As long as your render target is 64x64, pretty much anything goes.
Sprites tend to have artefacts if rotated at weird angles but this isn't against the rules or anything, this would be down to one's own design to remedy. I think you may have been referring to sub-pixels here? This only really happens if you're not actually rendering at 64x64... You may think you are, but some engines (like GameMaker, for example) actually render at window rez even if you've set everything to something lower. That's fixable but will depend on engine.