I had a guy offer precisely this type of unsolicited advice after spotting me on the bench and, honestly, it just pissed me off because he was making assumptions without getting all the facts. He assumed that because the bar position was skewed that the weight was a bit too heavy for me - though I'm wondering how he arrived at this conclusion after watching me bang out 10-12 reps. The reality is that I screwed up my left shoulder a couple years ago and it now tracks slightly differently than my right shoulder, which slightly skews the position of the bar. It doesn't matter if I'm doing warmups or heavy sets. Maybe he just caught me on a bad day, but at the time it really irritated me that he thought he was qualified to offer unsolicited advice after spotting me for several seconds.
That said, I have gotten advice from guys before and gladly accepted it. For example, several years ago a guy with massive traps was working in with me one day on shrugs and offered some advice that I still follow to this day. So I guess the real answer to your question is: It depends. If someone has legitimate, constructive advice to give and is basing it on an accurate observation, then I would certainly welcome it. However, if you watch me for several seconds and then determine that you are now qualified to advise me on anything, then I'd prefer you just keep your mouth shut.
As for me, I never offer anyone advice. If you take offense, then I have to deal with the awkwardness of seeing you everyday. If you readily accept it and get all chummy with me, then that's one more person I have to socialize with every damn day. I'm not anti-social or anything, but the gym is my time to work. I don't want to hear about little Jimmy's baseball game, how your boss is a dick, or how you think your wife is banging your best friend. I just want to lift.