It's really a personal decision. Some people get comfort and relaxation out of having a set schedule, so they don't have to think about what to do and where to go. Some people relax better when there's nothing on the agenda, and they just go with the flow. Some people like to land somewhere in the middle - they may know which park on which day, but not have a ride-by-ride itinerary when they get to the park. There's not really any wrong way to do Disney.
However, I would
always encourage first timers and infrequent visitors to do some research before they go, so they know what they are up against and can make an informed decision on whether to plan their trip in detail or just go with the flow. With all the travel books, and the Internet, there's no excuse for being uninformed. For example, the people who book the free dining promotion, and then don't bother to find out whether it's a good idea to make dining reservations before they go - a little research would really open their eyes!
I'm more of a go with the flow person. Whenever I'm going someplace new, I read up on everything in advance, and I usually have a few things in mind that I really want to do or see. But other than hotel reservations, and if needed, airline tickets, I usually don't plan more than that. Since I'm a frequent visitor to WDW, I rarely plan anything other than the resort reservation. I have my favorite rides I usually go on, but if a line is too long, I save it for the next trip. I will ask about walkups at TS restaurants, and quite often, I get in (even during free dining), but I have a backup plan I'm OK with if I can't. Things like
MVMCP I typically wait until a week or so before I'm going to get a ticket. I'm just not a planner. For me, being tied to a schedule, even a schedule as loose as having to be a particular place on a particular day, takes all the joy and adventure out of traveling. But I have some friends who are the total opposite, so I see both sides.