There are no such requirements. However, they do expect you to maintain a schedule of high caliber that demonstrates your readiness to enter and keep up with the pace of dental school.
But if you believe your GPA would increase if you take less rigorous science courses in 3rd and 4th year, I would take it slow and easy in order to maintain your GPA and upward trend. Especially in 3rd and 4th (if you're taking a gap year) because those latest grades will show your most recent academic achievement.