You definitely want the strap to go UNDER the roller, regardless of how you install the strap onto the winch drum.
As far as having the strap roll from the bottom or the top of the winch, I think it's better from the top. The reason being, not every boat ramp has the same slope, and depending on the construction of the ramp, the angle can change with different stages of the tide.
For instance, on my jetboat, I have the strap feeding from the top of the drum, routed under the roller, with the roller located above the bow eye when the boat is on the trailer.
At the ramp I often use, near the top of the ramp, at the high water mark, the angle is fairly steep. So, when I trailer my boat at high tide, the bow makes contact with the roller in the correct location.
BUT, farther down the ramp, near the low tide mark, the angle of the ramp is not as steep, and the base of the ramp is pluff mud, not hard packed oyster shells like the top of the ramp. So, when I trailer the boat at low tide, I often find that the roller is above my bow, so I have to stop short of running all the way onto the trailer.
Then I have to get out of the boat, and sometimes, I can put my shoulder under the bow and lift the boat while I winch it up the last foot or so. But sometimes, I actually have to get in the truck, and pull a few feet up the ramp, until I see my bow rise up, then I know it's above the roller, and at that point, I can put the truck in park, set the brake, get back out, and winch the boat onto the trailer the last foot or so (which is a little harder, because at that point, the trailer is mostly out of the water)
But the point I'm trying to make is that if I had the strap feeding from the bottom of the winch drum, it would pull the bow of the boat down farther, and complicate this cluster&$@# even more than it already is! #-o
