Well, it's not a "law" that you have to go back, and you know your own children better than anyone. here are some thoughts, though:
If the 5yo isn't used to walking 6-8 miles a day, he'll likely need a break.
If you have a stroller for him, and he'll nap in the stroller, that might be a good option.
It really depends on your overall schedule--if you're "rope-drop" type visitors, a nap is more necessary than "we'll get there when we get there" visitors. Also, if you have rest days or light days in your itinerary, daily naps might not be necessary.
If you are going in the summer time, it will be very hot. A nap or swim break would probably feel good for everyone. Also, you'll be able to enjoy more of the evening stuff if you've taken time to refresh. Travelling in off-season, this is less crucial.
We split up daily, but we have two older (12 and 14) and two younger (6 and 3). The 6yo will be taking a break daily. However, she may not nap every day, we're going in the summer, we're doing rope drop daily, and having a couple late nights.
I don't know if that really helps you. People have different styles and different kids. I'm envious of stoller-nappers! I would say, plan fo rthe worst and hope for the best.