Hmm, good question. I don't think I have any specific rules, but I seem to do it based on some notion of "distance": if the next or last is "a long way" from the top or bottom of the loop, I'll put in a label. Ultimately, it's a question of "if it looks better with the label, add it", but I can't quantify what I mean by "better".

On the other hand, I don't think I've ever used a label with redo (I don't use it very often 'cause I find it confusing. ;-)