I've seen them done both ways. Original Weissenborns have the saddle straight across. Some luthiers are now making them with an intonated saddle- I've even seen one done with a split saddle-see pic. Compare the attached pics of a trad weissenborn & a paddy burgin.
So, whadda ya think? To angle or not to angle? I dont have much luthiery theory, but to me, the same issues that require intonation on a regular fretted guitar are happening on a slide or resonator guitar too- just upside down. The string elongates off its straight path as you press with the slide=string elongating if you fret a regular guitar. You also have issues of thick string gauges and low bass notes, even thicker than regular guitar strings= requirement for intonation.
It seems to me that keeping it straight across may be 'traditional' but then we have all seen lots of old harmonys, stellas etc that had straight saddles- that doesnt make it right.