If you can, rotate your tyres front to rear regularly, so that all 4 need replacing at the same time.
This keeps the original front to rear balance.
But that doesn't always work out.
If you do need to replace only 2 tyres always fit the new ones to the front. On any car, FWD or RWD, since most of your braking is done by the front tyres, and you do want to be able to stop in the wet.