Sugarplum, *are* you using the built in styles? If so, you have nothing to worry about.
If not, there is *no* harm in using styles on your USB stick (as long as you don't keep re-organizing the folders). You can organize them by type (Pop, Rock, Latin) and sub folder the way Yamaha does to keep things organized. Try to avoid having more than 255 files in a single folder (as a lot of older Yamahas used a single byte to identify items in a folder, so you'd create an overrun error if you exceeded 255 items). which is why you *do* need folders, you can't just put all your custom styles in the root of your usb/user drive.
If your styles and registrations are in the same device, as Adrian mentions, you will also be fine.
Like I said, I now leave *everything* on the USB for portability, but accept that there are occasional slowdown hiccups. (if you do that, you *should* back up your USb stick every time you have a significant update, like a new collection of styles, finished a new registration, etc)
Mark