[wenz]

I am looking to buy a nice circular polarizer, probably a B+W Kaeseman MCR; though, I am open to other suggestions. I am debating about what size to get. I use my Tamron 17-50mm lens probably 90% of the time, which has a 67mm filter size. All my other lenses have smaller filter diameters. I was thinking of using step up rings for these other lenses. I am debating if I want to buy a 77mm filter and use step up rings even with my 17-50mm. Buying a 77mm would likely cover any future lens I would get (something like a 70-200mm 2.8), but I don’t think I will be getting one of these lenses anytime soon. What are your experiences using step up rings? Did you buy a new hood, if so what kind? These filters are expensive, so it would be nice to only have to buy (plus carry) one. On the other hand, I am not sure I want the hassle of using step up rings on the lens I will be using a vast majority of the time; plus, I think I would have to get and carry an extra hood and lens cap. Thank you.

[Kerrowdown]

Buy the best you can afford, cos hopefully you'll only ever buy one, especially if your going down the step ring route.

[Wombat]

We meet again.
I use a step-up ring to get from my 49mm M series lenses to the standard 58mm and haven't had any problems. I haven't bought a separate hood. The only thing to watch out for with a polarizer is interference with your shots. They're a thick filter anyway and adding a step-up ring is going to push it and your hood out there. Every setup is different, so check before you buy. In your situation I'd start with a dedicated filter for the lens you use 90% of the time and start saving for another for the rest of your lenses, if you really feel you need one.
With regard to brand, there's a review of three in the recent Digital Photography Black and White magazine. The B+W MRC, at 93% came out ahead of the Hoya (85%) and the Cokin (80%). The decider was the MRC coating on the B+W. It's more expensive, but, as Kerrowdown said, get the best you can afford.