I think if I am bothered or offended by something I should ask myself a few questions:
1. Am I pointing out someone else's rudeness or engaging in retaliatory rudeness, or am I maturely bringing up something that bothers me? There's a huge difference between telling someone she's being rude and asking someone not to do something that hurts, bothers or offends you.
2. Is this worth bringing up? Is it a minor or infrequent offense that I can get over, and go on with life as usual? If not, I think you ought to say something. I hate, hate, hate it when people want cookies for "not complaaaining," but they stew in silence and complain away with every action and sullen word.
3. If it's not something I can move past without stewing, can I bring it up politely? If not, I think you ought to wait until you've cooled down a bit. Things have a much greater chance of going south if the offended party is worked up when she tries to talk about it.
4. Is it an urgent situation? I think there are some cases where something needs to get done, NOW, and that might mean bumping someone's shoulder by accident. I do think every effort should be made to do this as politely and gently as possible. For instance, the time I was waiting tables and a guest who was traveling through (so, it would be hugely inconvenient for her to come back to the restaurant later that day, even) left her purse at the table. As I was cleaning up, I found it, and I could see that they were out in the parking lot, about to get into the car. I hurried to the exit, and there were two ladies ambling down the hallway side by side. I said, "Excuse me, please; I need to get through," several times, louder each time. I knew they heard me because they glanced back and gave me a dirty look, but didn't even try to move aside. I even explained, briefly, why I was in such a hurry ("Lady left her purse!"). I squeezed between them as carefully as I could, saying, "Sorry!" as I did so, and just made it to the lady in the parking lot before she drove away. I could hear the two ladies muttering about how rude I was, but I honestly didn't know what else I could have done. The restaurant was set up oddly, so the exit hallway was actually pretty long, and waiting behind them would have delayed me quite a bit.
I think a good, quick test of whether or not something is retaliatory rudeness is whether or not you're saying or doing something to get a rush of satisfaction, or if you're trying to fix a bad or unpleasant situation. Of course, that's not a perfect test; I get a rush of satisfaction from doing a lot of things that are perfectly polite. But I think it's a good starting point.