You can leave buttons out.. BUT.
If that button is required in a particular game preset, the game preset will not work properly if you did not wire the button.
For example, if you do not wire the A button, and then download the Sleeper COD5 preset with reload glitch, you will not be able to actually turn the reload glitch on and off, because it requires left trigger + 'a' button.
I would do all or nothing. If you just want plain old rapidfire there are easier kits to install. This kit is really for rapidfire + the glitch stuff.