Planning is in general a good idea. Planning family outings, or novels, or the weekly shopping, or means by which one may evade an unpleasant workmate is an excellent idea and makes catastrophic failure less likely.
But planning can slide over into causing failure, because it can be an excuse not to start doing the thing you’re planning. Planning a ‘perfect’ date can go on so long that you miss lots of chances for a nice regular one, and leave your significant feeling neglected. Planning a perfect dinner can take so long that you nearly pass out when your blood-sugar crashes because you were waiting too long to eat (as a certain person I know can attest). Once planning slides over into fantasizing, you may never do it at all.
And of course, planning can be absolute poison for the nervous writer. You can postpone your novel indefinitely while you do ‘essential’ research, or figure out the backstory, or wait on enough money to buy that book on Renaissance fashion that will just make the whole thing. In No Plot, No Problem, Chris Baty suggests taking a week to plan out your novel, but no longer, to keep yourself from being bogged down. This is for NaNoWriMo, of course, an exercise in on-the-fly writing if ever there was one, but I still think a limited planning window is a good idea. It could be a week, two weeks, or a month – especially if you’re doing historical and really do need to do your research – but I wouldn’t recommend going over that. You can always look stuff up as you go.
Planning is good, overplanning is bad, essentially. So I’m going to stop planning and go buy some shoes. Waiting for the perfect sale be damned, my plantar fasciitis is playing up now.