Since most of the interesting varieties of mint either don't set seed at all or the seed they set will not come true no matter *what*, cross-pollination is not an issue. (It doesn't affect the taste of the plant itself, just the genetic inheritance of any seeds that plant produces)
I would suggest keeping them reasonably far apart, 5' or more depending on how much you control your garden and how big you'll let the patches get, simply so they do not intertwine and get hard to tell what's what. That's merely for convenience, not for function, though
Personally I don't think apple mint tastes a lot like apples as such, but I guess everyone's tastebuds are different. Smell/taste the particular plant yourself before buying