jetis gave her response here, which got me to thinking.
I have no problem with an author being proud to be American, or Finnish, or Austrailian, or anything else for that matter and each writer has every right to write fanfic the way it makes them happy. I also don't see why people would want to change spelling from US to Brit (those being the English versions in question) since, like grammar it is simply a system for expression not the expression itself (if you see what I mean).
I write in US fandoms, but I still spell like a Brit and use grammar like a Brit since it is the method not the meaning and hence I think one should go with what one knows.
However, I do believe that there is an argument for going from one to the other for setting and characterisation, be it from US to Brit in HP or from Brit to US in just about every other fandom I'm in ::g::. I think some people can be far too anal about such things and take it too far, but I do think setting and characterisation are some of the most important things about fic, so Brit-picking and US-picking have their place in the beta system.
Am I making any sense?
What I'm trying to say is that I think the style of a fic is all about the author; it is part of the expression of creativity and should be left the hell alone (unless you are trying for something new and want someone to tell you what you're doing wrong), but the content, which gives characterisation (i.e. when the characters are speaking or giving the reader their thoughts) and the setting (i.e. the system within which they are living, what they eat etc) should be checked for whichever culture it is representing. It seems to me to be a fundamental part of the writing process. If you created your own character who was supposed to be a medieval knight and they continually said things like 'the best thing since sliced bread' you'd find them out of character since they've probably never seen a sliced loaf; as I see it the Brit-picking is the same.
For example, let's take Draco - he's a stuck up. pureblood wizard who if most wizards are anything to go by, knows sweet FA about Muggles. He does not watch TV and hence is highly unlikely to have been steeped in US culture even as far as the youth of today. Hence he is not about to slip into valley speak anytime soon, or come up with the latest grunge slang; neither is he likely to use Muggle idioms be they British or American. I made a gaff in GTS where he did just that which was kindly pointed out. How he speaks and thinks is all part of the characterisation and so I think Brit-picking and Muggle-picking are very valid.
Hence I believe that having your characters speaking incorrectly is as important to catch at the beta stage as having them do something which fundamentally breaks the rules of the universe. Also you could convince me that Harry might be eating a burger for lunch (with chips), but you will never convince me that meat loaf will ever be on the menu and there is definitely no coke machine in the corridor.
I think I probably just said what electricandroid said, just not as skillfully - oh well, I've said it now :).