I am a planning fanatic and I think it works well for our family. DH hates to wait in lines, so I plan to be at the parks before they open and to have a plan for where we're going to go and where we can eat. I don't always stick to it - I think one of the rules of planning is understanding that you might go off the plan. I'm not a lunatic about sticking to the plan, but it always helps me.
When I was single and childless, I didn't have a plan and it worked fine for me. It is important to note that I do not like any thrill rides. But with an impatient DH and a 2 and 3 year old, I now want to have a plan to avoid lines and crowded parks and have a nice place to have dinner at the end of the day.
DH laughs at my constant planning, but he is the one who points out people in the park standing around trying to figure out what ride to go on or complaining to a CM because they didn't make a PS at Crystal Palace and there's no tables available.
What I usually do is look at the best days to visit each park in the Unofficial Guide and then put together a chart for our visit with a row for each day and columns for breakfast, morning, lunch, afternoon, dinner and evening. I plot out days to visit parks and then try to plan where we will have lunch and dinner around which park or parks I decide for each day. (We usually eat breakfast in the room.) Since we usually go for 7 or 8 days, I sometimes map out what sections of a park we will focus on on a given day. This way, we don't go to Fantasyland first each time we go to MK or the safari each time we go to AK.
This method has always served me well. It's just one or two pages and I take a couple copies with me. If for some reason we decide to go off the plan for a day (it rains, DS has to go to AK AGAIN, etc.) I can call Disney Dining to try to change PS's or make new ones. The plan/itinerary also helps me put photos together and do scrapbooking when I return.
So if it makes you feel better to plan, then definitely do so.