Google seems to be very friendly to forum posts in general. Of course, that can be a double edged sword. I can't prove it, but they also seem to somehow focus on *negative* forum posts. I have one rare forum post for a product that happened years ago. When you put in the name of the product... There she is! Like a sore thumb. Even though I know there's a pile of praise posts... Those do come up, but old Mr. Negative is #1.
I think the hard part is controlling the negative stuff. If you attempt any deletion, most people will scream 'censorship'. It probably depends greatly on your customers... If you get a lot of cranks, I wouldn't do it.