It depends. If it's a condenser microphone then leaving it o/c will be fine. You might find that some AGC in the system winds the gain up - again it depends on system.
If you're concerned about it floating about then perhaps add a 1K5 in it's place (I've seen a few of condenser mic's are powered from 3.3V via a 1K5 or so), else you could put a capacitor of perhaps 100nF (risk it acting as microphone if you're unlucky).
Prob's just try it.