In all honesty, I don't think classes help that much because they go into way too much detail...spending extra time on practice problems like DAT destroyer would be more helpful...scoring well on the DAT is more about memorizing how to do specific types of problems rather than understanding concepts...
I took the DAT without having taken any science courses beyond the bare minimum requirements and still got a 25 AA and 24 TS just by studying DAT study materials...so if you have to take those science classes for your major go for it, it might help a little bit but don't take any extra challenging science classes solely to prepare for the DAT because it doesn't really help that much...
Take it whenever it fits conveniently into your schedule. If you've taken supper upper level bio classes prior, awesome! I was able to spend a little less time on anatomy and physiology, and more time on plants and taxonomy because I had just finished a human physiology class. That said, Barrons and Cliffs will get you where you need to be, regardless of your class track record.
idk how Biochem could help. I didn't come across anything Biochem related that wouldn't have been covered in other basic science classes. Genetics would help. I never took it. The depth of genetics that the DAT covers generally could be learned in your own review.