I agree that the least you should do is prime the tongues, but it is a wise idea to prime the rear of the boards as well. This will help seal the wood to stop the boards from cupping through moisture absorption, especially if you are attempting to cover up a wall that has a minor damp issue. Ideally all timber should be treated evenly on both sides. This is not always a practical option but I always try to prime the reverse of new skirtings, architraves, cladding etc that I fit.
Agreed with the advice here. I was always taught that the tongues of such boarding should be finished in the topcoat before putting together for the reasons DBM says. Sadly this detail seems to be very commonly omited nowadays on joinery and the like.