I don't bother stripping the existing LSP unless I have a *very* good reason do do so, and I hardly ever do.
The only reasons I'd do it are:
-so much layered LSP that it might cause issues with polishing (rarely an issue)
-contamination under/through the existing LSP
-changing LSPs and wanting to "clean slate" it so it's just the new LSP on ther
-maybe (and this is a stretch) if I'm trying to do an aggressive clay job with a very mild clay
-otherwise want to get down to bare paint for some specific [insert your own] reason