And please stick to IMU standards:
http://www.iab.net/standards/adunits.asp
Some sites/pages will be designed to promote specific sponsors, so they can be built around whatever the particular sponsor uses. Otherwise it can wreck a design to end up with extra white space in one direction or another. Equally it is frustrating to have to leave out someone's banners because they are just a few pixels too wide or deep.
My request would be for more 728x90 (IMU "Leaderboard") and 336x280 (IMU "Large Rectangle"). And for all the people popping up in these threads offering custom banners: if you are making them on request, why on earth not afterwards add them to your webmaster area so they are available for all?