I majored in neuroscience, two credits shy of a double major in an arts subject. I took everything from the hardcore sciences to astronomy to creative writing to French. There were bits and pieces that were helpful when I saw them again in dental school, but there was nothing that I couldn't have picked up from the DS lectures.
The other science courses I found helpful were the intro anatomy, physiology, and cell biology classes, which were prerequisites (for a reason). I found that the upper level stuff got really specific, and wasn't that useful. For example, I took an entire upper level course on cell signalling in the retina...Which probably helped in the 15 minutes we spent in DS talking about it, but I wouldn't recommend choosing upper level classes because you think you'll find them useful. For the most part, anything you need to know will be taught in dental school or in the prerequisite courses. If you don't believe me, ask for a calendar and check out the descriptions...You aren't going to find any upper level undergrad classes that prepare you for the meat and potatoes of dental school like drilling preparations, dental anatomy, occlusion, oral pathology, etc, and the first year classes aren't so detailed that you need advanced preparation for them to do well.
There is such a broad range of disciplines covered in dentistry that are mentioned that regardless of what classes you take, you'll probably find something relevant, and lots that isn't. Dental school has material that ranges from psychology to chemistry to business and everything in between, so no matter what you take you'll find areas that you know well and others that you don't. That's why I recommend taking the prerequisites and then filling your schedule with whatever else you enjoy.
This is the last opportunity you'll have to choose what interests you and explore all areas, so you might as well take advantage of it. You can approach education in one of two ways--You can see it as a means to an end, or you can get every experience you can from it along the way.