So the answer is yes, you could plant a sweet and a sour cherry together without having to worry that they will produce hybrid fruits by just getting pollinated (flowers) by another pollen source.

If you have limited space, you could plant multi-graft trees.

To the original question, I can think of very few instances where there would be any pollination-related disadvantages to planting one kind of tree near another. Growing some kinds of persimmons might cause other persimmons to grow seeded instead of seedless fruit. If you wanted to try to breed new varieties of fruit trees in any kind of controlled way you'd have to control pollination, but I assume pretty much everyone that's actually breeding new varieties of fruit trees in controlled ways does so by bagging flowers and hand-pollinating such that it still wouldn't matter what's planted where.

If you're a beginner, I think the short answer is that there really isn't anything you shouldn't plant because of pollination, but there are things you will need to plant for cross-pollination. A lot of fruit trees are self-incompatible, meaning the pollen from a tree won't pollinate the flowers from that same tree, so you'd need at least two separate varieties, generally from within the same species to pollinate each other. Sweet cherries and sour cherries are separate species, and so far as I know, won't cross at all, but maybe someone that knows more about cherries can say more about that. As harvestman said, when trees cross-pollinate that doesn't change the fruit; it only affects what kind of fruit you'd get if you took the seed from that fruit and grew another tree and it fruited, and generally speaking that's not how any of the common tree fruits are propagated anyway, so it's a non-issue for all typical backyard or commercial growers.