it's going to be something that's variable by school...just send an email and ask them.
chances are, if any of the courses you are taking is a core course like bio, chem, orgo, whatever - you'll probably need to send in an update.
otherwise i would say that if you got accepted somewhere in an early round, then no, it doesn't matter, as long as you pass everything by the time you graduate. if you failed courses and didn't tell them, an offer could be rescinded.
the issue is if you are waitlisted -- you don't want anything to hinder your movement on the list....however, if you've already interviewed and the pre-fall semester grades were good then what's done is done....i'm doubtful this semester (short of failing) will have any major impact.