It can't hurt to treat it before you add. If you're worried about a product being effective, get a test kit that doesn't test for ammonia/ammonium, just ammonia (one of the big box producers makes one, can't remember who). Like stated above, prime won't get rid of it, just convert it to ammonium, which only harms fish at much higher levels. A few bacterias (that we call nitrifying or "beneficial" bacteria) will convert ammonium to nitrogen by-products, so they will be consumed too.
Why stress the fish? Add it, can't hurt anything. Plus prime will take care of the chlorine, chloramine, and other heavy metals in the water so you could take care of everything in one fell swoop.