It totally depends on wear, but if they're bald then i'd replace them.
I would look at Kumho Ecstas 4X - They are the only tire I could fine that had a treadwear rating of over 400 and AA traction.
I had them on my 2003 530i and they were freaking amazing. Quiet as heck and gripped the road as good as a Continental DW. (Had the DW's on the car. Good tires but louder than the kumho's and the wear sucked.)
I'll be putting them on my 335 when I need new tires.
2010 Alpine White 335i
- Sports Package, Premium Package, Navigation, Comfort Access