Modern fabric softeners are usually a highly penetrating micro-emulsion. The aim is to penetrate the weave of close-knit fibres, even resistant ones like nylon and some polymers, and to lubricate the fibres thus making the fabric feel softer and less prone to building up static. Having said that can we infer that the same thing would happen with wood fibres which are more tightly interwoven then the closest fabric? It's a possibility, but my feeling on this is that anything which reduces the surface tension of water will make it have a more wetting effect and consequently allow for greater penetration. I have always believed this to be the object of using water in bending; the steam generated makes the fibres more plastic and this is made easier by good penetration. It's quite likely that a squidge of detergent might increase the penetration although without the lubricant effect. Only one way to find out...experiment!
I tend to use plain old water with my side bender, but after wetting I wrap the sides in foil so that the steam generated is contained rather than evaporating. One caveat.. once the sides are bent they need to be removed from the foil and the sides put back in the bender with a short burst of heat to dry them out fully.