Don't have any experience with 870's but when I replaced my Mossy's it was tight and reinstalled the new one just as tight. I figure that it shouldn't be overly tight or overly loos either. I'd go with a tight fit not so tight that you have to lean in to it but just enough torgue to make sure its snug and it shouldn't wiggle any.
Well I put it kinda tight and now racking the slide is much more difficult. The stock fore end came off and I installed a hogue. It didnt go on tight, so I guess I need to loosen it up some.
That may be the case every manufacturer is diferent and have diferent specs. Best bet is to set it where it doesn't effect the operation but maintains integrity(don't want it coming off in the middle of a shoot).