[mwigdahl]

Depends. If your schedule has a lot of flux (meetings being cancelled ad-hoc, etc.) then it can be useful to accept everything but notify the host that you probably won't be there unless something changes. That way if something does change you at least still have it on your calendar (I use Outlook, maybe other calendar/meeting apps handle this better).

[nraf]

Google Calendar has a Maybe option that solves it simply enough.

[disgruntledphd2]

Outlook has Tentative, for the same reason.