There is something to be said for the excitement of not having any idea what a mountain may hold. On the other hand, many people will probably want to be guaranteed, at least sometimes, that they will have all their rivery/chasmy/magma needs met at the outset of the journey.
A nice compromise may be to have the degree of knowledge of the mountain available when choosing a site along with wildlife and climate and whatnot Depending on the algorithm Toady uses to generate terrain, this would be hard or easy. Only he knows (give us the source, please Toady! (-8) Lots of algorithms that generate terrain start from the general (Is there a river here? How high generally are the hills?) and work toward the specific. If this is the case, it would be relatively easy to implement a system where the Dwarves could be more or less certain of the contents of their site. Perhaps even the "richness" of the mountain could be a property of the site. I don't know, Toady has done a fantastic job making such decisions and I'm sure he'll come up with a sensible solution.
As sort of an aside, I don't know how much Toady cares about what his players suggest on this forum, but the fact that he reads and responds to our incessant demands and suggestions is an absolute wonder. Thanks Toady.
The future of the fortress is bright indeed!