Sealer slows, but won't stop stains from being absorbed. All grout except epoxy can benefit from sealing. You don't need to be all that careful when sealing. The only cardinal rule is don't let it dry on tile, as it will leave streaks. Put some on, let it soak it, then buff it off before it dries. Depending on the grout, you may have to do this several times before it won't absorb any more. There may be slight variations, depending on the brand and type.