I basically throw caution to the wind and hope things come out in playtesting. That being said, I try to eliminate as many glaring balance issues as I can. I shoot for a target to balance against, and I try to set the power level to that standard, and after that I try to close any loopholes. The fact is, most casual gamers don't look for ways to break the game, or the RNG, or ways even to make their characters unusually strong. They just come up with a concept, look for the options that match it, and go from there. Since this group of people represents the majority of the player pool, I don't usually mind if I left some loose end that horribly unbalances the game (especially if I don't know that I did).
Min-maxing also has its place, and I don't want to completely discourage it when I design classes and tweak rules. If someone wants to take the time to tweak their character's performance to be a little above the curve, that should be fine, but it should have an upper limit, and this limit should be reigned in enough so that such a min-maxed character doesn't completely outshine other PCs who don't have the time or desire for it.
EDIT: You should also maybe change the title of the thread. It's misleading. Sounded to me like, "What company do you design for?"