Writing for Americans
billswift: As long as they are the standard spellings, it shouldn't make much difference.  It's the unexpected spellings, especially the ones that make your readers stop and wonder what you are trying to say that are problems.  Anyone who reads much will be as comfortable reading the British spellings as the American.  I use some spellings interchangeably - I am more likely to write "grey" than "gray" and I spell "defence/defense" either way (it is possible that I have even mixed them in the same essay - which is generally a bad idea).