If you do both sides then I would say no, I didn't but that doesn't mean I did it right either. Using the seam sealer is just added protection but you may not get as good adhesion with the bedliner. Mine was blasted, primed then coated. I used sealer where it wasn't lined and then painted it. The sealer will work best where you have larger gaps and the liner won't get into the void, otherwise I would skip it.
Jim W.