As usual, I agree with BDL and disagree, or the reverse.
You DO want to bring stock very slowly up to temperature -- taking 45 minutes or so for a big pot is good. The reason is that as the proteins in the meat come up in temperature, they coagulate -- this is most of your scum. If you heat very slowly, they will coagulate on the surface of the meat, under the water, and not float free. This means that you can extract their flavor without their clouding the stock. The faster you raise the temperature, the more you are in effect doing a little light stirring, and as (I hope) you know one should not stir stock.
An example: you've got perfectly clear stock simmering. Now reach in and poke a piece of meat. See all the stuff that floats up? That's coagulated protein that's lightly stuck to the surface of the meat. You must now skim that off to keep your stock clear. If you don't poke the meat, you don't have to do this. And if you bring your stock up to temperature very slowly, so that it does not roil significantly, very little scum will float.
I believe that floating scum also becomes bitter because of oxidation, but I'm not positive about that. Certainly it clouds stock, which is enough reason to get rid of it.