Planning is...well...hard to do, to a certain extent. As many have already said, players are quite the fickle beasts to where they end up doing things you never once anticipated in your plannings. And if you never planned with those things in mind, everything gets thrown out of whack.
That said, I've done planning and used it more as a guideline and general outline to work under. Taking into consideration, Grimm Nights. The first incarnation of this RP there was like...ZERO planning involved. I had a general end goal that I wanted, but never fleshed anything out...and I honestly believe that's what sort of had that first version die within two pages (or less, I can't remember). Planning may not be a bad thing in an RP, though always be flexible when doing things. Having the flexibility while going by a general plan for the RP is always a good thing to do.
Cause life can happen and they may not be able to post, so you'll need to be flexible in having to put a character or two in limbo until they return... Or...if players end up going in a direction that is one thing you weren't expecting. Always be as flexible as you can be with an RP, cause if you can roll with the punches, things will be a lot easier.