FT vs. ST

John...Thank you for the invaluable resources you provide through CFFB and Talk To Me Johnnie. After reading the post “Legs Too Large? WTF?” I began to wonder about utilizing the 20-rep max. In the paraphrase, Mark Rippetoe uses the example of an 80% 1RM for a set of 20 reps. My question to you is: do you recommend using a 20 rep max from time to time on the squat and if so, how often? I imagine that doing a 20-rep max using about 80% of 1RM would put the legs out of commission for a good 2 days...