I never really plan out my team beforehand. In certain games I'm already sure of which starter I want to choose, but that's not really as much planning as it is preference, however I always plan to catch the starter bird unless I'm doing some kind of challenge, so I guess that's the furthest my planning goes. Otherwise I just go on with the games and catch whatever falls in my interest and that I want to use on my team.
Very few times have I actually planned out a team beforehand, and I guess one of those would be with my most recent character on PokeMMO, where I caught several Pokemon that weren't accessible until after the E4 (Houndour, Mareep and Teddiursa if anyone was interested :p), traded them over to my new character, and just went on with the game. I'd already planned to catch a Pidgey and a Mankey, and I never got a sixth member. I didn't start using them until I'd finished the second gym though, since most of them were close to or over level 20, and I wanted to make sure I was far enough ahead with badges so they didn't just start ignoring me at some point, so I was only using Mankey and my Charmander for a while, which was gonna be replaced by Houndour afterwards :p But yeah, that's pretty much the only one I can remember.