Their familiarity will slowly decrease if you don't scout it, but it is very slow compared to how quickly they learn it. So yes, if you are at 100% you don't gain familiarity and might consider not scouting it.
One caveat, though, is that you still get a boost beyond familiarity if you face a play you scouted that week. So if your opponent uses a play a lot you still gain by scouting it even if you are already at 100% familiarity. If you are at 100% on a play that you don't expect to face, though, you are safe letting it go for a little while to gain familiarity with other plays.