Recent experience:
IAD-LHR-IAD. No pre-boarding announcement. Walked right up with business class pax and asked to board with no issues on either flight. Some have mentioned that they like to board last. I can't fathom doing that at this point (though my daughter is not yet mobile). I like the idea of splitting up with one parent pre-boarding and getting everything situated.
Personally, I have found that carrying on three bags +/- a car seat is far easier if there are no other passengers in the general area. It takes us about 5 minutes to get the bags where we need them (both in the overhead and at our feet) and to get settled with the little one. 2 to 3 minutes of that is in the aisle... I used to think that pre-boarding was a perk for the parents, but after travelling on an airline that didn't allow it - I now realise that it benefits other passengers much, much more.
My apologies again to the gentleman we abused with elbows from BOS-BWI.