To clarify one thing, I wasn't trying to say that getting autographs is a bad thing. I started collecting cards when I was 9 and I still have a memorabilia and autographed card collection. Interestingly enough, I don't like "hand autos" much and I'm actually annoyed that some of the items I acquired were autographed (like my Jake Peavy BayBears road jersey).
I don't think it's inherently wrong to ask for autographs, just be aware of the environment. If a sign says no autographs... then perhaps you should pay heed to the sign. A introduction that asks for a autograph is not a polite introduction. Even if your time is short don't start out a conversation with "please sign this" you're generally making yourself a nuisance. If they don't have time to talk but are not already signing autographs may be a wave or word of encouragement is the best greeting. Remember, we live in the Ebay era. They don't know you, they don't know if you plan to run to Ebay with the autograph they just signed for you.
I remember at the end of one concert when a lot of people ended up on stage, I approached a artist I had communicated with several times (email, chat, sit down meeting, going out to eat together, etc...) and just wanted to say hey. He didn't even acknowledge me. Upon reflection, he was in performer mode. You can't afford to pay attention to individuals during that time and if you do it's to protect yourself from them (they have a nasty habit of pulling hair and the like). I couldn't be annoyed at him ignoring me, I had inadvertently came into his office, sat down and tried having a conversation while he still had work to do.
As far as getting autographs in person, just try to consider the person. I never asked for a autograph upon initially meeting a celebrity. The times I did ask for one it was at least on the second day of being around them. There's also several celebrities I have been around that I never asked for a autograph from. Believe it or not they are people, if you just enjoy casual conversation with them why turn it into something else? The same goes for taking pictures with them, it doesn't mean you shouldn't. But perhaps you shouldn't run up to them with a camera. If we remember they are people, not objects perhaps we can do a better job of treating them accordingly.
Do you stop to acknowledge everyone you pass on the street? Do you stop and have a conversation with a homeless guy when he stops you to ask for spare change? I remember someone stating how they gave a homeless guy money almost every day, but the guy got mad the one day he didn't... make sure you don't come off like that.