You should definitely sign up for RR, no matter how you book your trip. Even on your first, trip, it allows you to pre-check-in and get your mini-bar stocked prior to arrival, and you also have the option of taking advantage of a Trading Places day, so you can check out the other Couples resort in the same town (CSS/CTI, CN/CSA). If you are a returning guest, depending on the length of your previous stay(s), you begin getting gifts, resort credits, and invitations to the Repeater Dinner.
If you want to earn rewards on you repeat trips ( you will be a repeater!) Sign up and if you want to do a Trading Places at the sister Couples resort in the same town you can. It's painless to sign up.