Creeping up on a stranger is never polite. If you see riders ahead and are tempted to ride their wheel, pass first to say hello, and offer your own wheel for a minute. If they take yours, then they should let you take theirs.
If someone passes you and you want to ride their wheel, again, say hello as they go by. Personally, whether I grab their wheel or not depends on the nature of their response and how "pro" they are. As long as there are no aero bars involved, anyone in race-fit pro team kit is just asking for it.
But if they are in club fit jersey or street clothes and aren't friendly, I let them go.