IMHO the key is how much contact there is between the head and the edge. A sharp edge (such as a double-45-degree cut) will have minimal contact, and will allow more sustain and (as I understand it, and based on my experience) will result in a slightly brighter-sounding drum. A rounder cut will have more contact between the shell and head, and will produce a "warmer" sound with a bit less sustain. Often, builders will "split the difference," cutting a sharp double-45-degree cut, but lightly sanding the edges to round them off a bit.
Here's a page from SMD Drums (a custom builder), that presents a variety of bearing edges, with pictures and descriptions:
http://www.smddrums.com/bearing_edges.htmThe double-45 seems to be the cut "du jour" - everyone seems to offer that one these days. The toms on my old six-ply Ludwigs (not THAT old, just late '70s, but not the "classic 3-ply vintage" early-70s-and-earlier Luddies) have a bit of a rounded outer edge to the cut, and they've got a wonderful, warm sound. I think that if I had to choose just one shape for my bearing edges, I'd lean towards something like these - a 45-degree inner cut with a rounded outer edge.
One interesting possibility I've heard proposed, but I've not had the chance to try it: some sort of rounded edge on the batter head, for a nice, warm attack without excessive overtones, but with a sharp double-45 cut on the resonant side to...well, maximize resonance.
I've heard that proposed as a nice middle ground between "modern" drum sounds and "vintage" drum sounds. Again, I haven't tried it, but I sure do like the logic.
