Oh goodness, don't kill yourself by taking GEs all up front and packing your last couple years with nothing but science classes.
There's a reason some science classes are a credit or so more than other classes: they're time-consuming. The last thing you want is to be taking four science classes with labs all in one semester because you didn't spread things out and got in a position where you had to take them in order to graduate on schedule.
Because I am a non-trad who had to go back to school for the pre-vet requirements, I was only taking science classes (my undergrad was, of all things, theology - you can imagine how many science classes I took). I can attest to the fact that it can be a pretty big bummer to pack them all together. Doable, but not as fun as they could be.