I think you should always agree on the name. I'm finding it really hard thinking of a boy's name this time round too (just like you not ttc till Jan but I like to be prepared!) because OH doesn't like any names I suggest. We both fell in love with Austin as soon as it came up as a choice, so it was pretty easy. We've got a girl's name too (Isabelle(!)) which we both love, and a couple of back ups if we change our minds. It's just boy's names that are so hard!
At least there's time to think of one that we love as much as Austin.
Did you both like your DD's name when you picked that?