With invitations, if I know I can't make it, I'll say so. It depends on how far out the event is for a "yes". If it's on Saturday, I can give you an answer. If it's next month...not so much. I'll say something like "I should be able to make it, but I'll let you know otherwise", and try to remember to give them a definitive "yes" closer in.

Jen12, I was brought up the same way. Be a little early. For stuff with friends, I'd rather be a little bit late. In my group of friends, it's understood that 8:00 generally means 8:10 or 8;15.