It depends how old they are. If the tread depth is still really close to new, it's OK. If not, it can be hard on the dif because you have 2 tires always spinning at slightly different speeds. If they are front on a rwd vehicle, I think you can have alignment issues if they are different enough. If they are rear tires on an FWD car, it matters the least, because they roll independently. But you are going to rotate your tires eventually, so even if it is OK in one situation, you will be putting strain on the dif after rotation. AWD cars are even more sensitive, and often need all 4 replaced.
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