as long as your not over speeding the engine. if the engine was designed to run up too say 10,000 rpm as long as everything is fine the valve train won't float tilll you exceed 10,000 rpm . hypothetically if in 4thgear at say 5,000 rpm and you down shift to 2nd and it brings engine speed up to 8,000 rpm you shouldn't have any valve float but if tapped out in 5th and you drop into 1st that is a different story. the main thing is the how high the rpm,s get when you downshift. since I really don,t know the rpm ceiling of the WR and /or your particular riding style /rpms you are hitting I cant say whether it is ok or not.
personally I don't think that engine braking causes any more wear than on the gas except for maybe the back side of the knobbies. If you are beating the snot out of it on the upside or the downside you're still beating the snot out of it is my opinion