Bio wheels are said by some to be nitrate factories. They don't produce anymore nitrates then any other bio filter, but they can't reduce them. They do a great job of reducing ammonia and nitrites and help to aerate the water but to reduce the nitrates, the water has to get into the DSB or live rock. When the DSB or rock breaks down ammonia and nitrites the nitrates don't have far to go to reach the anaerobic areas. So the problem with bio wheels is they are somewhat innefficient at de-nitrating. I'm not saying don't use one, I'm using one right now, I'm just explaining what they do. They are great at reducing ammonia and nitrites, though. If you have enough sand and rock to properly filter your water I'd leave it off. I don't have enough, I'm getting more rock this week, then I'll likely remove mine after a couple of weeks with the rock.
One other thing. There'll only be as much bacteria reducing ammonia/nitrites/nitrates as there is "food" for them. A bio wheel can make your live rock defficient in bacteria because with such a highly oxygenated area the bacteria will prefer to live on the wheel. I'm not sure if this will hurt or help, some say it does.
I'm not an expert or anything, this is just what I've read about biowheels. So keep that in mind.