I made derailer and derailler synonyms for derailleur back in September, 2010 (I can see the tag synonym history).
I believe at the time my reasoning was simply based on which version of the tag occurred most frequently on the site. English language Stack Exchange sites don't support tags with accents, so "dérailleur" with the accent isn't possible. It's also easier to merge the less popular tag into the more popular tag than to do the opposite.
Outside of tags, I believe all three spellings (not derailler, though) are acceptable and there's no need to edit posts to change the spelling.
Personally, I buy Sheldon Brown's argument for "derailer" being the preferred spelling. The way it's pronounced in English is totally wrong for the French spelling. In my posts, I'll use "derailer", but when editing another post, I'll stick with the spelling that's in use.