Membership, currency and attracting custom

This is a general question that I hope one or two of you might of had some experience of and can possibly advise me.

We are about to launch a number of sites using the membership plugin (which is working really well for us), However, on one of our sites we have a split number of UK and US members, as well as other dotted around the globe.

Do we use UK, US or Euro currency? Does this affect who might join in the future and want to pay?

I don't know if it could be a future feature of membership to have a currency conversion...?