So far as rudder/nosewheel steering servos are concerned the nylon gear types are best. The plastic will absorb a lot more shock the a metal gear servo, and standard servos are fine, as Bax and RFJ allude. Use a yellow nyrod inner for the steering bit and any major shock will simply deflect the rod and leave the servo alone. My models (Atlas, Curare, Superstar, Sicroly etc) have only flown off grass and have been doing so for over five years with no servo problems at all. Consider to, that taxiing around the field is not the major part of model flying, and takeoffs and landings are all done with the nosewheel off the ground anyway, so there is little need for much steering, and very little wheel movement (left and right) is actually needed. You have probably seen trike models darting all over the field on takeoff and landing, a sure sign of way too much steering movement. Rudders steer airplanes, not wheels.
Evan, WB #12.