I always try and make an effective character which means I always try and min/max to some degree. What that degree is depends on the rules, the setting, the GM, and the game.
If I'm playing D&D, I'll be digging through books and supplements and handbooks and all sorts of things. It's a very combat heavy game and if you build your character poorly it'll bite you in the arse and your group. So the extent of my min/maxing is great.
If I'm playing BESM2e with my main group, it's less of an issue. Decisions are made much more in the context of "What makes sense for my character?" than "How do I become the biggest baddest character ever?". Keep in mind though that as it's a Point Buy System that is designed to work with any genre it's really open to abuse, even if you're not trying. If you don't make those decisions based on your character you're really minimizing the fun of yourself and everyone else and maximizing the suck. It's a roleplay heavy group as well, and we might only have a combat every three or four session. Granted those usually last two and are epic in scale. But it's not the focus of the game at all. Without even trying hard my character has the greatest potential damage of anyone in the group, even if the character is a bit of a glass cannon.