I bill on average outbound, always have. In every example I've ever seen it is to the advantage of the client - especially if they do TGP posting.
Look at this guy that posts movie files on TGPs:
He peaks at upwards of 60 mbit but his billing on average is in the mid twenties.
He doesn't want his server capped or throttled, he wants high performance all the time so that he can handle demand when it's there.
Those that bill on average will charge more per MBPS and those on 95th will bill less per MBPS. Sometimes the overall difference is negligible, sometimes you will find a large savings with one over the other depending on how the hosts are priced. When you start talking about purchasing more than just a few megabit, buying on average can save you substantial money most of the time.
Cheers,
Brad