Normally I don't care at all about our opponents, but there are some reasonable limits you can set, should you feel inclined. When we played Leyton Orient in the cup in the L1 promotion season and thrashed them 6-0, I guess we could have been 'ole'ing' and taking the piss at the end, but at the same time it would feel a bit like rubbing salt in the wound of a club whose supporters have been through a heap of crap, and a bit tinpot to celebrate so wildly against a team so far below us.
If we had that result vs Man Utd in the FA Cup, then you'd expect that the piss taking would be at maximum, and couldn't really complain for it being so.
Teams in the same division or higher its a free for all as far as i'm concerned. This may be just personal to me though, and others may not agree.