Welcome aboard, Alecia! Your fourth Disney cruise is coming up, and since you're a Castaway Club member, you must really be excited, knowing all the fun that is in store for you on your cruise!
It certainly is relaxing to eat dinner out on your own verandah, so I completely understand that. My family, however, does love to get to know our servers, as they really become part of our cruise experience, but since you've cruised before you know what you like best.
Gratuities are always at your own discretion. But keep in mind that the
traditional gratuity that each server receives is really part of what makes up their pay, so they do count on that income. You might not dine in the main dining restaurants for dinner, but you will be eating somewhere throughout the cruise, whether it be a restaurant at lunch or breakfast or the buffet, and the same servers work all three meals, so at some point you're encountering someone serving you.
I'd suggest still tipping the traditional amount, but again, this is always up to you. If you know you won't be dining in the restaurants for dinner at all, I'm sure that your service staff would appreciate the heads up, so you might want to let the head server know the first night, or even note that on your reservation so they aren't preparing for you each night.
Let me know if you have further questions as you continue to plan your fourth Disney cruise, Alecia!