It depends on how the one-night-stand went. If it was some sort of life-altering experience that brought down the walls of human relationships with a single fuck, then yeah, talk to them. If it was one of those times where you have really good sex, then talk to each other for hours after, and don't make each other violently ill in the morning, then look; you made some sort of connection that might be worth exploring. I'm not saying you should pursue a relationship at all; I'm just saying that if you had a grand old time with that person, then acting approachable, friendly, and conversational when you see them out might benefit you later. On the other hand, if something just got stuck in something else and then you went to bed in a pool of what you hope was water, fuck it. If you see them before they see you, have an escape mechanism planned out. My personal favorite is getting out my cell phone out and calling my grandma, who I keep forgetting to call, because there's no better time than the present (when you don't want to talk to someone) to reconnect with your elders. How the hell is she anyway? If the sex was that bad and you left feeling like it was a mistake, then it probably was. You would have never talked to each other in the first place if it weren't for that tequila shot or seven you took last night, so why start now? I'm not telling you to be an asshole to anyone, I'm telling you that if you see them coming, and by some miracle of god remember who they are, just casually call your grandma. Know what I'm saying? However, for those unavoidable times when they spot you first, and are staring straight into your soul trying to figure out where they know you from, (you ate whipped cream out of their butthole last night), then by all means, be nice. You know this. If they're the ones that initiate a conversation, let them do the talking, be polite, and smile even if seeing them activates your gag reflex in a very unnerving way. They probably already feel shitty about going home with you; so don't make them feel worse. But whether your one-night-stand was good or bad, I feel like I shouldn't have to tell you that if you see them, you should be as nice as humanly possible and try to contain the puke that you puked in your mouth when they entered your view, because you're an adult human being aged 18-35 and that's what they do.