Laying out some tile. Carpet will be replaced after the tile.

http://tinypic.com/view.php?pic=244s70h&s=4

Is the carpet typically fastened to the riser (glue or tack strip?) then
tucked behind the tile edge. Or does the carpet sit on top of the tile then
maybe some kind of molding at the junction?

Tucking behind the tile would seem to be better looking to me but maybe
because there's not much depth to tuck it tends to fray or something.

Tile is 3/8" thick, riser is wood, tile base is slab.

I'm asking because I need to know whether to butt the tile againtst the
riser or leave an amount of space for tucking. If so, how much space?

Thanks.