I'm going to be at both DL AND DCA for 5 days from December 5-9. Is it really necessary to create a plan?
Necessary? No, especially not when you have that long. We were there for four days, and I did make plans beforehand, just to get a sense of what I should prioritize for the first hour or two. But after that first hour or so, we just monitored wait times and winged it, planning on the fly around whatever Fastpass(es) we had at the time. Usually we were holding at least two at any one time; in one case, we had five at once (RSR/WOC/2 Buzz/Indy).
I agree with @WDWRDpro. We did something similar when we visited DLR. We used it to know what we should prioritize, but after that we used our TPs more as a checklist to be sure we didn't miss anything that we wanted to do.
I've never done a full day plan for DLR (nor have I ever been fortunate enough to have had more than 3 days). I pre-plan which park I'm going to start in and the first 3 or 4 attractions that I plan on doing at RD, but after that, it's pretty much winging it (unlike WDW which seems to "require" OCD planning months in advance). A caveat to this is I've spent close to 50 days at DLR over the years, can navigate the parks with my eyes closed, and have the "feel" of what to do when. I also don't necessarily have the need to "do everything" every time I go. One of the beauties of DLR (compared to WDW) is that you can walk from the gate of one park to the other in about 3 minutes, so hopping back and forth is a breeze. Plan your park days around the EE schedule. If you have access to EE, USE IT. If you don't, start off at the "other" park.
We regularly go for 5 days and rarely make a plan. Like others have mentioned, we usually only plan the first 2-3 hours, especially for rides that will be impacted, but I don't use the touring plans, I just make a priority list for ourselves. For example, when we do early morning at DCA, we always plan to do Toy Story Midway Mania, because once the park opens it goes to 40 minutes quickly. We usually will ride it 3 or 4 times before we move on. The only other thing we try to plan is if there are any reservations we want to make sure we have. But even with those, I find that with only a couple of exceptions (Ariel's Grotto and the Blue Bayou) we can get same day reservations without a problem.
I'd make a plan for sure. There's so much great food that I'd have a plan for meals. And, I'd plan what park/s I will go on each day while considering early entry and- as others have mentioned- a well thought out rope drop plan which can include World of Color fast passes if I'm not doing a dining package. I'd figure out my evenings too even beyond parades and spectaculars: some attractions should be experienced at night. And, I'd make time to explore DTD and the resorts including drinks at Trader Sam. And, I'd even consider carving out a little time for LA.