For the purpose of QACing, I have found that changing my C stick to do specials works for me. While not really doing anything for QAC-Jump combinations, it's awesome for doing QAC-QAC combinations (stuff like the QAC lock, etc.), because you don't have to constantly mash your D-stick (or whatever the main analog stick is called ~_~) at +1 priority speeds. It's also nice, because if you mistime, and buffer a QA with your B-stick before Pikachu is actually in the "floaty position", ready for a QA cancel, you'll get a jump instead of just lagging.
I also like B-sticking with Pikachu so I can do things like stop sliding when I do a dash-thunder (nice for those high flying kills sometimes, also unexpected compared to jump>jump>vB, or just a sliding Thunder) via wavebouncing the Thunder on the ground. And I'd also like to say that having tap jump on isn't necessary, and harms more than it helps (imho). You can just mash the jump button, and simple jumps aren't that hard to time out of a QAC anyway.
I also have R set to jump, and X/Y set to grab (I only use L for shield anyway, and it's nice to be able to jump then immediately do aerials instead of moving my fingers like a... like a Brawl master). I use the classic controller, but it's similar enough to GC (I don't do this out of preference, my GC controller was worn out and I couldn't find a new one. I have to say it's ok, and the left analog stick is in a better position, but it's unfortunately smaller, and big fingers mess up when you're trying to go to the right while wavebouncing a Thunder). I'd also like to hear some other people's controller configurations, not just for QACing, but for playing with Pikachu in general.