I don't know in detail, I've never encountered it except with second-generation WLP833 (bock lager yeast). As I understand it, most ale yeasts produce little enough and/or consume diacetyl fast enough that it's rarely a problem. Lager yeasts are a lot more likely to produce it, and don't consume it rapidly at low temperatures, hence the rest. Most ale fermentations don't bother with a rest, partly because they are already fermenting in the mid-to-upper 60s.
If you don't taste the off-flavor in a sample, it's not going to be produced during a cold crash, so there's no need to do the rest. It won't hurt anything, though, so if you're paranoid and can spare a couple days, there's not really a reason *not* to do one.