I plan some of them beforehand. There's usually one or two I absolutely want in my team due to simple preferences and the like. The rest however, I tend to just wing. I usually, but not always catch the "vendor trash" pokémon of any region, mostly because of my need for some early extra manpower.
It's a habit of mine to always try and get my favourite of that generation in my team. So Breloom from Hoenn, Slowbro from Kanto, etcetera. As for the further details on movesets, I pay some attention to it, but not as much, since I hardly ever devise some masterful tactic in my head. And I don't even pay attention to the stats, and IVs, and all those things I don't really know what they do to begin with, at all.