most of my bikes have horizontal or semi horizontal rear dropouts. when i pedal, especially uphill, i am putting a LOT of force on the rear wheel that is desperately trying the pull the wheel forward. but it won't because the QR is preventing it. i would imagine front wheels have less force on them.

it just so happens that the other day i switched tires on my bike. from 20mm to a 28mm. guess what?
the tire hit the inside of the fork just underneath the brake mounting hole. i was surprised. anyway, i just mounted the wheel a little short of "home" in the fork ends. haven't had any trouble with it for several hundred miles.