I've never tossed a knife in an ultrasonic cleaner, but I've used them on a lot of other things with great results - sounds like a reasonable plan. You probably already know the cautions, such as using either a cleaner sold for the purpose or something that can't harm anything that's going in. (For example, some Simple Green formulations will strip anodizing.) I think I'd avoid letting the temperature get above what you'd be comfortable putting your hand into. When things come out of an ultrasonic cleaner they tend to be VERY clean, so relubrication (or using one of the lubricating solutions as a final step with the ultrasonic cleaner) is a good idea.