I waffle on this subject a lot: are you really "inconveniencing" someone whose job it is to serve you? And if you are, does it matter?
When this question inevitably comes up in a restaurant or bar, your friends probably all have differing theories. What's surprising is those who have worked in food service aren't always the ones wanting to give the waitress a break or tip the bartender extra just because. Out of habit, I usually leave generous tips but sometimes I catch myself - I'm just about as broke as a server and if the service was bad (truly bad, not just that I went somewhere insanely busy on a Friday night and had to wait five minutes longer than normal) I will tip exactly 15 percent, possibly less. I will sometimes not ask for something because it seems like it will inconvenience somebody, but then I remember - that's their job. When I was serving, I would much rather have someone assertively ask for (or remind me of) what they need rather than pull that passive-aggressive bullshit of "Oh, I don't want to bother her. Now I'll just be quietly upset all night that I didn't get extra salad dressing, and tell everyone later that the service was bad."
I was visiting friends in Brooklyn this weekend and while we were walking back to the train a little before 2 AM, we (unbelievably) couldn't find any late-night food options in the neighborhood we were in (I know, not even pizza! In Brooklyn!). Drunk and hungry, we finally stumbled upon a hot dog place. They closed at 2, and one of my companions and I declared we didn't want to be "assholes" and go into an establishment a few minutes before closing. They were probably cleaning up and wanting to go home. Our other friend, annoyed with our passivity, reminded us that the restaurant was open until 2, and made a living off of providing food for people. It's their JOB. Distracted and drunk, we wandered away from the hot dog place but I thought about it later - she's right. It's their job. In fact, I just got pissed off at a bakery in a small town a couple weeks ago where I went in a half-hour before closing and basically could not get them to sell me anything because it was all "put away." How annoying.
On a side note, the friend who reminded us that the hot dog people make a living off of selling hot dogs, and would probably be glad to do so even right before closing, is a former server and even worked at the same restaurant as me in high school. She also once left a condescending biyatch waitress at some crappy tapas bar in Uptown a bunch of scattered change on the table as a tip after she was excruciatingly rude to us.
Proof that fairness doesn't always mean accommodation, even if you're trained to think that way.