Where you get the MBA matters if you're not going to practice medicine but rather compete in business per se, and it's your first business job. If you have experience prior to the MD/MBA, it'll matter alot less (if that experience is useful, of course).
Also, there are certain types of jobs that the MBA won't matter for if you also have the MD from a top school - think consulting here. The top consulting firms hire ALOT of non-business advanced degree types (so an MBA from a mid tier school won't matter). Where you go from there is up to you, as these jobs are often considered the "second half" of an MBA and confer all the prestige you'll need (if you're good).
Over time, what'll matter is how good you are and how hard you work. It may take you a bit longer if you don't get the prestigious starting job, but you can get wherever you want if you're good enough (yes, even with only an "OK" MBA).
Work. Work. Work.
P