I find mine follows the camber of the road. So sometimes it'll want to pull to the left if the camber slants to the left and other times the car will want to pull to the right again if the road slopes to the right. On a totally flat level road it'll head straight as a dye.
Mine doesn't do it either, but I've read various complaints about it, particularly in the States.
Some say its incurable and call it a 'characteristic' - fault, I'd say - others claim to have had it fixed.
I read all this ages ago, so I can't direct you to a particular thread - it was on one of these - you'll have to search - Link C32life Link MBWorld Link Benzworld