I always go under. Never had rain issues as I use drip lines. To me, It seems to put less stress on the tarp to go under as you don't have to stretch the tarp so tight to keep it from sagging in the middle. Guess it's all about preference.
I agree about the stress and I don't like the sag in the morning when I use the tarp under the CRL, so I always run the ridge line under the tarp.