[saghm]

To be fair, a decent number of people also overcorrect and use "Tim and I" for the objective case as well (e.g. "The teacher told Tim and I that our grammar was incorrect"). I think simply giving the correct construction for the case that the speaker actually used isn't sufficient for using proper grammar; one needs to be able to identify which case is which to be able to use "correct" grammar.

[taylodl]

The real fun is when someone corrects you for saying "The teacher told Tim and me" and then you give them a grammar lesson to the use of the objective case. I'll admit to a little bit of smugness when that happens.

Otherwise no, I don't correct people's grammar unless they've specifically asked me to review something.

[billforsternz]

Nicely put, I didn't realise it but I am, as you say, overcorrecting in this way myself. Interestingly I have a friend who has quietly tried to point this out to me (a counter example to the greater point) but you have explained it better.