Shin Okada
Explorer
i'd be inclined to think that this (taken from the d20srd):i trust my players fully, but i think i'd want an active hand in what goes into the mix, as some weird combination of feats, skills or classes might make something unbalancing - not just for the campaign, but within the group. i stressed that these are basically like sidekicks and shouldn't really outshine anyone within the group itself.
I don't worry much about this.
A cohort is at least 2 levels behind a PC. And by default, a cohort has elite array ability scores and it is often inferior to PC's (I remember in multiple vote threads in EN world, so many DMs were voting that they were using 32-point buy or similar method). Also, as a NPC, a cohort has inferior starting gears comparing to the PCs of the same level.
Regarding builds, if your players have skills and tendency to max-out character builds, they must be doing it on their own PCs already. So cohorts will not outshine in that aspect, too.