Of all those I've had (40+), most were flat. I think radiusing of the shelf is a fairly recent feature if it's on newer bows...
Well, drat.. y'all started me wondering so I got up and went down and checked out the Hills I have left. All the "Craig" Hills are flat as to the horizontal surface. The leather is centered and acts as a bit of a rise in the middle, but there is no radiusing on shelf itself. There is slight, and I mean SLIGHT radiusing of the vertical arrow plate surface.
On my Kramer Hill there is definite radiusing of both surfaces and I recall that this was also true of my Schulz Hills. Both Kramer and Schulz did a lot more sophisticated riser shaping than was true before them or since.
For those who really want to go to the source, Howard's bow is dead flat on both surfaces, the shelf is very narrow, even by today's Hill standards, and as far as i can tell, it never had any leather... it was shot right off of the wood. I shoot it that way and it performs fine. When I built Flapjack, I tried to copy the construction of Howard's bow as best I could, and I am also shooting this bow right off of the unpadded shelf. At first I thought, "this is going to kick the arrow all over the place..." but it doesn't. I shoot Flapjack quite well.
Hope all of that helps a bit.
Oh, yeah... that other question... yes, I like Hills....