Both ways have their advantages and disadvantages.
Over the Front:
Pros: Smallest footprint when open, better front-rear weight distribution
Cons: Doesnt look as cool, no covered standing room under open tent, need to fab up a spot for the ladder to sit, have to stand on hood to close and open
It all comes down to your needs and preferences. My truck is very rear heavy and moving the tent further forward would have helped. In the end, I decided that having a covered area behind my truck was more valuable, so I mounted it to the rear.
A leaking fuel can shouldn't really be a concern. The covers on all the tents that I have seen will hold up to a little fuel being spilled on them, but really, if you are worried about your fuel cans leaking, you should find another type of fuel can. Ideally, the cans would also be mounted much lower. 10gal at 6-7lbs/gal is a fair amount of weight to be carrying up top, especially when added to the load of a roof top tent.