If Snape and/or Lupin appear alive in a fic set after the war...
I want to see an explanation of how he/they survived his/their canon deaths.
An explanation (even just in passing) is nice, but not a necessity.
I'll just take it as read that he's/they're alive. Don't even care about how.
He/They can only be alive if it's Alternate Universe (AU).
If Snape and Lupin appear alive in a fic set after the war...
No special labels or warnings are necessary.
Warn for "non-canon" or "canon through book X" or something like that.
It must be labeled AU.
Other (will elaborate in comments!)
As for Remus & Tonks (and Teddy)...
Can we just pretend it never happened?
Let's pretend it never happened, *and* we don't have to call it AU.
I prefer fic to be consistent up through OotP - then he got out of there.
Up through HBP is fine - as long as Remus doesn't have a kid.
Teddy should only be around if he is central to the fic.
Just leave it as it is after DH, Teddy alive, and then deal with Remus being alive separately.
If you don't call it AU, then you have to mention Tonks before pairing Remus with *anyone*.
I'm just tired of reading about how Remus left Tonks before I get to the Snupin-y goodness.
I don't care about Remus *or* Tonks, really.
Other thoughts - details in the comments!