I think that primarily, they look for a good trend in grades, so dropping a little isn't great - but that also depends on what you mean by "little". Ex/ going from a bunch of 3.8s and 3.9s to a 3.6 isn't a huge deal.... going from 3.7 to a 3.0 isn't great.... going down to below a 3.0 is pretty bad... but if you can explain it away, you should be okay.
I think what's most important is to have good grades in all your science classes, especially the pre-reqs and at least some of your upper level courses. A friend of mine had B's for both semesters of general chem and general chem lab, but then had A's in both semesters of organic chem, organic chem lab, and in biochemistry - and I don't think anyone cared about gen. chem.
What school you go to is also taken into account, but I really don't think that, for ex., a 3.2 from Yale is better than a 3.8 from Penn State - if the GPAs are very close, then the school may make a difference.
Hope this helps!!!
- Quid
