I like my center on top. My Sony RPTV has a sloping top and I ended up building a small shelf out of a piece of MDF and some shims to fit the top. I velcroed the shelf to set and velcroed my center to the shelf. Works, but it's a pain that Sony doesn't design the cabinet with a flat top like most other sets. My B&W LCR60 center only weights about 20 lbs. so no worrys about weight. I think I would worry about trying to balance a 50 lbs center up there, though.
You have to get creative. If you use velcro, a tip is to get the "industrial" stuff from Home Depot -- it's much heavier and has better adhesive.