it fully depend on the person, alot of people dont like being tickled, so no matter what time you choose to do it youl probably end up making things worse not better, and for someone who does like it, if your lucky enuf to know, i guess the right time would reveal itself out of common sense,
personally i beleive there are far better ways to cheers someone up, altho i guess it depends to what degree theyre upset, for example, a girl mate of mine was cheated on by her long term boyfriend the other week at a club we were in, they then had a huge argument and with that the relationship came to a close, i went outside to talk to her and she was really really upset, now she doesnt mind a tickle or 6 but it ws the last thing on my mind when it came to attempting to cheer her up, i think if someones truely upset about something important, then they just need support rather than tickling.