I am not involved in MBA admissions, just working from general experience here.
- Grades have diminishing importance the longer ago they occurred, I think. If you are not concerned about how your grades would affect your immediate prospects upon graduation, then I think this should limit concern about how grades would affect your prospects for an MBA program 3-4 years after graduation. What you do in those 3-4 years will be the proximate (not sole, just proximate) determinant of your MBA prospects I think.
- Consistent with this thought, perhaps you are looking for an undergraduate experience that, apart from grades, will prepare you to do special things immediately after graduation, and that you are looking for this to be the cornerstone for your MBA candidacy. This is not the place to get into all the advantages Brown has to offer in terms of the Econ, BEO, Engineering, and Applied Math/Econ concentrations...in addition to the atmosphere of self-direction and personal responsibility created by the Open Curriculum...but you can easily research them.
- Whatever impact your grades would have for MBA admission 3-4 years after graduation, you can see from just reading this little thread that grade devaluation, if any, would be greater for Harvard and Yale than for Brown. I cannot speak to the devaluation factor, if any, for other schools.
If an elite school has the ideal concentration (major) for you, and the fit is right in terms of geography, social life, feel, overall curriculum and whatever factors matter to you, I think it would be a difficult decision to turn that school down because you read on a message board that another school has less grade inflation