ONLY replace them if they are leaking or if you are having problems. I have seen some Subarus go for 200k with out any need for new head gaskets. I have seen many Subaru dealerships recommend this when there was just the slightest hint of an oil weep from the head gasket, it is a scam in my opinion. The cars that I inspected are still going strong, 50k after the dealer said the head gaskets needed replacing. If the car is over heating, then yes do them but be prepared to pay about $2500.
you can find the range on our estimator below, click on service and go to engine and head gaskets.