Forget the screws, go with POM rivets.
They're plastic rivets that you install with a hand rivet gun, takes all of 3 seconds to install and/or remove.
Since they're not vastly stronger than the plastic they're securing, they won't rip the mounting tabs off your fenders if you roll it or hit something on the trail.
The rivets will pop off and save the plastic, while the screws would still be holding that mounting tab to the frame after the rest of the fender is ripped off.
I've verified this by installing a junk piece of fender to an old frame and yanking on the fender until something broke...and after several attempts the plastic rivets consistently popped off without damage to the mounting tab.
They hold the fenders every bit as securely as a screw would in normal circumstances (they're not loose and they don't flap around on rough terrain...rivets by definition are tightened securely when installed).
To remove them, all you need is a punch or screwdriver to push the little stem into the rivet and it'll loosen the rivet enough to be pulled out by hand.
You can find the rivets and rivet gun at any harbor freight, the rivets are CHEAP and come in several sizes, the gun is plenty tough for the task, and you won't have to worry about using big washers that leave marks on brand new fenders.
They'll last several season, or indefinitely I suppose, as I installed them on my LTR and they're still every bit as snug as the day I installed them.
Also, more of a personal preference, but I think the plastic rivets give the fenders more of an OEM look than steel bolts.
Buy it online here:
http://www.harborfreight.com/hand-riveter-with-40-industrial-strength-poly-rivets-97757.html
