I wouldn't be suppressed if I have taken 10,000 frames with my 24-70 and have never noticed anything change at different focal lengths and I use that lens at f2.8 quite often. Actually the only lens I've owned that had issues that couldn't be readily fixed was the 24-105 which distorted too much when shooting wider than 35mm on my old 5D. But most of the L lenses that I have owned could at times seem just a tiny bit less crisp when stopped down all the way. Nothing horrible to stop menfrommdoing it if I really needed the depth of field but I don't normally go there.
