Howdy,
This might be super necro(?) but I'd like to give my input on tap jump floating as an old school melee Peach main that just started playing her this year in Wii U.
If you've ever seen Armada play, then you've seen how crispy he gets low float to (frame perfect) Nair and then FF to float cancel (giving him no landing lag upon landing the float aerial.) In melee, it's really good pressure to be able to quickly float at any height and to disengage immediately after activating your aerial all to abuse this cancel. I don't think he claws anymore, and I believe he uses Y.
I used both Y and X to float at any height:
X: I used to use X by default back in the day and claw my index finger vertically across the button in order to have access to the C-stick with the thumb, thus easier Uair and Dair while moving forward or back. NOTE: This is the same effect Brawl and Smash Wii U Peach players want when they assign L or R to jump. I have long pianist fingers, so YMMV.
Y: I used this mainly for any empty jump movement or auto-canceled aerial that could end in a float or instant low float. This is because you can hold down Y with the top of the thumb and "piano key"/roll down to the A button with the bottom half of the thumb to get Nair, Fair, and Bair and it allows you to release Y after pressing A, or keeping Y held down after pressing A to continue the float and drift in or out.
Ironically, each button correlates inversely to the axis direction it's most optimal for (i.e. X button for easier Y-axis attacks, and Y button for X-axis attacks). All of this said, I like tap jump so assigning different buttons and turning it off for me is convoluted when I have all of this (frame perfect) muscle memory tech. But to each their own

My 0.02