This seems to have come up time and time again - but seriously, does it really matter? Are we not spending time over the wrong things here? Should our effort not be focused on more important things such as those 7 essential meta questions floating around, attracting more members to the site, posting good, useful questions and answers and so on?
As long as it's clear (which "Raspberry Pi", "Pi", "RPi" all are) I don't see the need to enforce a particular preference - and let's face it, that's all it is. Yes, I think blatantly redundant uses of "Pi" (in whatever form) should probably be removed, but let's not start automatically editing titles just to impose an arbitrary preference. There's much more important stuff to focus on.