We wondered this same thing on our most recent trip with a 5 and (just turned) 7 year old. The first two days were fine - we did not rent strollers. On day three we wanted to do a longer day with a lot of park hopping so we rented two strollers that day (they are 2 for $25). It worked out well because they were tired from the two days before and also we could take them to fireworks and let them crash in the stroller if they were too tired. (The night before we had to leave WOC early b/c they were falling asleep and we wished we had rented a stroller for it!) The last day was a half day so we again went without strollers. If you plan to do a bunch of attractions/rides in one area in the morning, and then again in the afternoon, it is pretty easy to go without. (For example, we spent one morning entirely on one side of DCA - Soarin', Animation building, Muppets, Monsters, Aladdin; and another on the other side - Golden Zephyr, Goofy's Sky School, Symphony Swings, Jumpin Jellyfish, Little Mermaid, etc.) Since we stayed in one area for several hours it was not too much walking for them and it was in smaller spurts, so it worked out great! Maybe just rent strollers on those days that you really need them, like when you plan a long day or want to do a lot of parkhopping.
Have fun!