As far as the science ones, I'm fairly sure you have to take it for a grade, either way, a Pass = C in the eyes of Adcoms. As far as the non-science, I think it would depend on the school, I'd call the schools you're interested in.
Each school is different, but if it is a core predental class then you might have to take it over for a grade unless you can take a reasonable substitute.
To answer your question with respect to non-science classes:
I took 6 hours of English Pass/Fail (almost all schools had a requirement for 6 hours of English), and I didnt have any problems...
I doubt the same would fly with Bio/OChem...
I wouldn't pass/fail a dental prereq... I did pass/fail a graduation prereq (Spanish) so that I could focus on my sciences, but I wish I hadn't, because it would have actually boosted my GPA. Oh well. No one seemed to care that I pass/failed Spanish.
I wouldn't pass/fail a dental prereq... I did pass/fail a graduation prereq (Spanish) so that I could focus on my sciences, but I wish I hadn't, because it would have actually boosted my GPA. Oh well. No one seemed to care that I pass/failed Spanish.