I build a few single rigged saddles every year.3/4 or 5/8s,the main concern is to make sure your tree fits.proper spread and the right rock to the bars.Have not had any real problems with saddle movement.The fellow that runs the BLM wild horse unit in Litchfield,CA,rides a 5/8s single rig and say it stays put even ropinrg large horses.
Steve