Personally, if you plan all working all your shoulder button usage into one, I'd suggest switching it all over to whatever you currently use to shield. Shielding is the most reactionary usage of those buttons, so whatever finger you're using for that probably has developed the most twitchy reflexes for that button. Everything else will be easier to switch over- wavedashing has a few frames warning for your brain to correct your fingers (no, not the left one, the right one!) as does L-canceling, etc. But shielding is something you want to come out on frame one of your reaction time, so however long it takes your brain to correct yourself while learning to apply this is how much slower your reaction time for shielding becomes, whereas the same isn't true for things that have one frame or more of start-up. I used to L-cancel with L, everything else with R. That was like, over a year ago? That finger still occasionally twitches for l-canceling. You probably don't want to switch your shielding button to a jump button for that reason- even if you successfully shield with w/e you DO switch to shielding with, if your old shielding finger twitches, you'll cancel your shield with a jump and take the hit during jumpstart.