Good question that should get you an amazing array of answers. My two cents, which buys very little, unfortunately:
I've done extensive tripping both on season and off. We sometimes have a plan. Other times, we have no plan. For the most part, we have something that's less of a plan, but more of an agenda.
For the first time, we had a definitive touring plan, written out so we could follow it. There is no doubt, especially on your first trip, that you will get more done with a plan. You just can't get around that. So my personal answer would be for a first time visitor, I'd say yes, make a plan.
However, you are going during a somewhat slow period. Are you planning on going back a few times? If I were you and really, really didn't want to do the whole regiment thing, I'd take the following tact, what I call making an agenda, rather than a plan (I guess they're the same thing, but not to me).
We now set up the days we're going to parks, avoiding the EE and traditional heavy days (example, avoid MK on Saturday at all costs, much better on a Wednesday). We make our priority seatings for those days. We decide on an off day or two, if we have the time. We then make out a list of attractions that we just don't want to miss, no matter what. Use early mornings and late afternoons, plus fastpass, to achieve your goals of riding Space Mountain, Test Track, Tower of Terror, or whatever suits your fancy.
From there, accept that you're going to miss some things. Use your park hopper (you do have park hoppers, or are going to get them?) To get out of parks that are too much for you. In early to mid-May (before Memorial Day), if it's any day but Saturday and you're in a packed park, there's an unpacked park somewhere that's more reasonable. And in an unpacked park, you'll see more, at a more leisurely pace. And consider taking a mid-day break. That's when it's the hottest. We always leave the parks after a lunchtime PS (usually around 2 PM or so), and we head back around five after a rest/swim in the pool/whatever. We find ourselves refreshed and ready to go, and it's nice to file past the tired, sunburnt masses who are leaving about then because they just can't take it anymore. And the lines drop at bit around this time, even in the most crowded parks.
The disadvantage of this strategy: you do have a tendency to not "do everything," but you're not going to anyway. The advantage is you do less, but you see more, if that makes any sense. And in the long run, you'll see most everything you want to. This is how we do it now. The planned trips of the "old days" we'd have a set plan taken from the various tour books and info found here. Regardless of which plan you take, get up early (most people don't), take a break mid-afternoon (most people don't), and just enjoy being there (everyone should).
A final tip: when all heck is breaking loose, it's almost always less crowded at Epcot, just because it's an enormous park with so much more to do.
Hope that helps!
Pat