TENTEN > HAIR
Tenten's hairstyle has two brown buns of hair on each side of her head. She also keeps some of the loose hair down over her headband.
As a ninja, I guess she don't want her hair to interfere with her attacks, which might be a reason why she puts it up. (seeing she's a weapons master) Who knows, maybe someday we'll see her with her hair down. ;D though I must say I like her current hairstyle.
Other hair-styles
In the 4th opening, Tenten is shown with the other Konoha kunoichi - Ino, Sakura and Hinata. Here, her hairstyle is slightly different. It's twisted together and put up behind with two ribbons(which is somewhat hard to see), making it look like her usual buns(Imagine hair inside those rings and see what you get XD). Even though this was shown in the intro, it was never shown in the series.
Tenten is yet again shown with another hairstyle, only this time in the 6th ending of the series (again, nothing else than that). It's not really much to explain about it, seems to me it's just the one from the 4th opening untied. ^^;;