I don't know about specific angles, but I've got my highbacks at just slightly more angle than my boots. My personal opinion -- and I'm no expert -- would be that a little less heel overhang is better than a little less toe overhang because the torque you can apply with the highbacks more than makes up for the lack of leverage at the heel. You still have the "weight not centered" problem, but given that we naturally adjust our balance anyway, that's probably not going to be a killer.
If it's a choice between using the disk holes to get just the perfect stance width or using the disks to get just the perfect centering though, I'd go with centering.
Everything's compromise though. You'll never get it perfect. Plus you're not stuck with your first choice. I've re-mounted my bindings while on the mountain before because i just didn't like what I started with. Just don't drop one of the bolts.