This question certainly is quite subjective, it's really all about your style, desire to walk a lot, energy level, and normal habits (are you late night people, early birds, etc...). If this is your first ever trip to WDW, I'll tell you...and I think most others would agree...that you cannot fully grasp the size of WDW until you see it. It's massive, and deceptively so. You don't realize how much you're walking until your legs start to get tired. I also think that no matter their style, a majority of people here would agree that it's not the best idea to do 7+ days of 10 hours per day in the parks. I'm sure there are some who can do that, but most don't recommend it. I don't either.
However, having said that, the only way we plan a full day outside the parks is if we're going to make the drive to Daytona Beach. Other than that, we're in the parks every day, even on a 17 day trip. We didn't come down to see a condo complexs' swimming pool or the bedroom. We're there for WDW. So we're in the parks every day. However, our "down" days will be shorter hours in the parks. For example, we may do:
Mon: 9-4
Tue: 9-3
Wed: 4-11
Thur: 9-4
Fri: 11-2
That's just a random example, but it's the concept of what we do. So in that example, Friday is only 3 hours in the parks. We don't start until 11am, so that allows us to sleep as late as we want. We have FP+ at 11am, noon and 1pm. After that, then we'll head back to our condo (we stay off site) and take a nap, or relax by the pool. Then maybe take the kids out for dinner or dessert, come home and watch a movie or maybe lightly hit Disney Springs...i.e. park closest to the stores we're going to and leisurely walk around a little.
So in my opinion, no, you don't need a full day away from the parks. But I don't recommend trying to do an entire week of heavy park days.