Every time I play a pokemon game for the first time, I just throw together whatever seems good at the time I catch it. Then if it ends up learning good moves when it levels up, I keep it on my team.
When I start a new file, I do pretty much the same, except I try to get different pokemon together than the ones I used previous times.
Also in the 4th Gen ones I did the same thing, until I realized after I got my butt handed back to me in pieces after a Wi-Fi match that I needed a better team, so I researched pokemon's Max stats and movesets.
Another thing I don't plan ahead is pokemons nicknames, which I always come up with on the spot when I catch them. I usually try to make them make sense or be funny, for example: An Electabuzz named "Ampere", or a Teddiursa named "Pedobear"