at 170,000 miles I think its good insurance to put a fresh rear main seal in, especially if you plan on keeping the car for awhile. They actually dont have a reputation for leaking the crankcase cover can though. I changed my rear main seal at 87,000 when I did my clutch just to get the experience, and for piece of mind. I didnt want to do $700 in clutch parts to have a leak a couple years later.