Mini rant
Most American slang is known well in Europe via tv, books, movies.
This is true for one simple reason -
other English-speaking countries don't tend to change other people's language - not even for children. I read American as well as English children's books, and that's how I
learnt American slang. It's called broadening your mind...
The Harry Potter changes seem horrific to me, because they subscribe to this insidious myth that Americans are more stupid/less broad-minded than other people, and the poor things need more help. It is NOT TRUE! Americans are as capable of understanding other forms of English as anyone.
I once did a TV co-production with finance from Paramount about a serial killer in London. We had a terrific US executive, but even he was forced to ask us to dub some of the English words into US versions for the US release (including, yep, 'tap' into 'faucet'). We even had to change 'telly' into 'TV' - because 'Americans won't understand'. The exec was as offended as we were by the implication his countrymen were that ignorant, and was thoroughly embarrassed by the whole thing. He himself spoke fluent French and (I think) Danish as well as beautiful English, and resented being treated like a xenophobic moron.

The problem we have now is that it's self-perpetuating. Because other countries have the respect to leave US English alone, but the US changes other English to its own, eventually Americans
will become ignorant to other people's speech. Already, even on that pit of misinformation firstwriter.com there's a weary disclaimer saying that they are NOT spelling words like 'honour' and 'colour' wrongly, and that these are genuinely acceptable versions in a country called the UK.
Even worse would be if we in the UK started making the same kind of changes. For all I know we already are (it's a long, long time since I was a child) but if we are I think it stinks... Channelley, your remark:
when I buy a book set in the US from the UK, the language is UK English
suggests it's already happening here. Were these books just badly written by UK writers, or actually edited versions of US originals?
Sorry, rant over. But it upsets me, and I know it upsets a lot of Americans.
So, to answer your question, Channelley, I guess it's up to you. If you're already doing a US version to send to one agent and there's no extra work involved, you might as well send that one to all the US agents. It's quite true your
publisher will sort it in the copy edit, but an
agent won't. It's good to hear waylander had no problems with it, but there are so many excuses an agent can look for to reject something - why give them one you don't have to?
Louise