As a general rule, you should definitely pick one facial feature to emphasize when applying makeup.

If you want to go for a strong mouth, play down the eyes with simple makeup (as Katherine Heigl did at the Emmys).  I find neutral eye shadow, with eyeliner and mascara does enough to bring out my eyes, without me looking like a lady of the night.

However, if you’re going for a smoky eye, then I’d recommend sticking to more neutral shades for the lips.

The only time you can get way with both is for evening functions.  Even then, it’s all about getting the right balance so that you look sexy and alluring, and not like Bozo the Clown.