Should you seal grout when using RedGard Backing?
You don't have to.  It is completely up to you.  Sealing the grout is doing absolutely nothing as far as water-proofing your current setup.  I Redgard/HB many showers/tubs and here is what I do:


unsanded - gets sealant.  In my opinion it stains easily, especially below level where water constantly hits.
sanded - don't seal it.  It looks better unsealed and this grout allows you to "sand" out a stain.
epoxy - no sealant of course.