Calling flop bet with small pocket pair +EV or -EV?

[LockLow34]

Hypothetical...

You limp in middle position with 55, several callers. Flop comes A94r. There's a bet and 2 callers, and you can put at least one of them on an ace.

Is it +EV or -EV to call this bet, expecting to hit your set on the turn?

I've had several situations like this recently where I've had hands like 33 or 55, mucked after the flop when I didn't hit my set, then the card comes on the turn that would have given me a set. I just want to know for sure I'm doing the right thing by mucking after the flop.