OP here--I'm very interested by the responses! Let me clarify a bit. First off, we were opening presents so early (before DD's first nap) because of MIL, and only because of MIL. She came into town at dinnertime on Christmas Eve and only stayed until 10am. That was completely on her and her boyfriend's schedule, and in fact I was (privately) upset and disappointed that she stayed for such a short time. If she had been staying longer, or hadn't been there at all, we would have opened presents later in the day after DD's first nap (or done a present or two, and then the rest later). She made it clear that she was here to see DD open presents for her first Christmas.
What frustrated me was that we were having to rush and open presents before 8am because of her, and she made even that less than enjoyable. I would have much preferred to have stretched presents out the rest of the day. Believe me, I completely realize that 3mo DD had little/no clue what was going on, and in fact I felt a little silly wrapping presents. But, by the same token, it's important to DH and I to start our family's Christmas traditions, and why not start now?