Yeah, flat is usually fine... although on a "noticeably radiused" top (no, I can't quantify that... but it's known when seen) I'll partially radius the bridge to meet the top halfway. In other words, just help it along a bit with a slight radius and let the remainder flatten-out to help tame the top down.
I am of the opinion that the bridge should usually have some arching (I'm talking 20 feet or more). I agree with Mike that a bulging top should be met halfway.
In any case, a plane and a scraper are really great tools for shaping the bottom of a bridge. A well tuned standard angle block plane like a # 9 1/2 or a # 103 with a razor sharp blade does most of the work and the fine tuning is done with a scraper. A straightedge is also helpful for monitoring your progress. In the past I've used a razor blade as a scraper (give it a burr with a screw driver shaft) but a few months back I invested in a couple of good scrapers which both hold a burr longer and are easier to control than a razor blade.
To hold bridges for planing, I drilled a couple of holes through a common aluminum caul and I compression fit two tapered dowels into the outer bridge pin holes. The thumbscrews supporting the wings of the bridge may be used to help join the bridge. I tighten the thumbscrews to support the bridge ends for a flat cut or loosen the thumbscrews to allow them to deflect away from the cutter creating a concave surface. Spot planing can be done at a forty five degree angle across the grain or with a radius-soled plane or a scraper.
I don't know much about luthiery, but I'm making my first guitar right now and working on the bridge. I used some self-adhesive sandpaper applied to the sealed (with a 'spit coat' of shellac) top and sanded the bottom of the bridge to mate perfectly with the top by moving the bridge back & forth in rapid, short strokes over the bridge location. Took a little while, but it worked perfectly to fit the bottom of the bridge to the domed arch of the top. The sandpaper (150 grit) lifted back off with no problem. If the top were more arched than mine, I can see roughing in the arch with a scraper or plane, and then finishing it with the sandpaper.
Not meaning to hi-jack the thread, but what are you building, Mark? I completed my first build in the spring of last year: a StewMac 000. It's not a great example of woodworking, but it's a fine sounding and playing guitar.
My first guitar is a small "concert" size, short-scale steel string with a cutaway that I've aspired to build for over 30 years (that's how long I've had the wood). You can read about it and see some photos on my frets.net blog. I'm currently applying a french polish finish to the body and fashioning the ebony bridge.
The top has a good bit of radius to it. If I leave flat then I can not get the ends of bridge to lay flat for gluing. The bridge plate is new. I don't see how I could ever get a flat bridge to work on this top. Nathan's remarks sound the most useful. I have also thought about using Mark's method.
There's more than one way to skin a cat so all of our working methods should be taken with a grain of salt.
That said, I have some concerns about using sandpaper for final fitting a bridge during a bridge reglue. 1) The particles from the paper could easily damage the lacquer if you're not careful. 2) The best glue for this type of job is hide glue and remnants of sandpaper particles and sanding marks are a hindrance to a hide glue joint (probably not the best for tite bond either...). 3) There's really no room to move the bridge accross sandpaper on the top. That sort of thing works on a lacquered mandolin where the bridge sits on top of the finish but on a guitar, the elevation of the surrounding finish and the inevitable .005" of top thickness loss necessary for cleaning up the spruce leaves an indentation at the bridge's footprint that would prevent that method from working.
What you say is true for a reglue if the top is fully laquered already, unless, like you say, one is really careful, (which one should always be!). I used 150 grit and shaped the bridge after only the sealer coat of shellac was applied to the whole top. I marked the bottom with a pencil, to be sure I had sanded the whole surface when the pencil marks were gone. I guess it wouldn't work if there was a .005" void of finish where an old bridge was being replaced. One would have to strip and refinish the whole top. I'm probably being naive, but the sanding marks on the bottom of my bridge blank seem negligible to me. I'll be using titebond. I won't be fastening the bridge down untill I finish french polishing the top and then scrape off the finish from the footprint of where the bridge is applied.
Hi leparker-- This is the way I do my bridge plates on a new dred. and I think that if you are going to replace a bridge it is done the same--
Take a piece of 150 grit sand paper and mask it to the top of the box where the bridge is going to go, and then run the bottom of the bridge plate over the sand paper and it will conform to the top after a fashion. Run the bridge back and forth from the tail to the neck.
Take and also mark the bottom of the bridge plate with a pencil to see when it does conform..when the pencil mark is gone, you are done.
Just my 2 cents.. good luck with your fix...
I use the same method as Donald, and I think it is essential to get the foot of the bridge to be a good fit to the dome of the top. No danger to the top of the guitar to place the smooth side of a piece of 150 grit on the top and then rub the bridge around on that surface. You will need to take stock out of the middle, until the edges are touching the soundboard. Eyeball it first and if there is a lot to be sanded off you can get started with a scraper or (gently) belt-sander, then finish it off with the sheet on the top. Like Donald said, when you think you are close put lots of pencil marks on the bottom of the bridge and rub some more to make sure that all points are being sanded off.
The only problem is that the radius of the top will change depending on the humidity status of the guitar. Just make sure that it is not too dry when you do the fitting.