It just comes down to your preference. I used to load up my 400 Foreman with homemade 8ft wooden ramps by driving it in, plus I had a canopy on my truck which made it pretty tricky, but it can be done. If your ATV has a winch, then you can just use it if you're a bit leery of driving it in.

 

Years ago I had a Honda 500cc three wheeler. I horsed it into the bed backwards. That way when I got to the woods I just dropped the tailgate, fired her up and jumped it off the tailgate (never did get any ramps).

Load it however you like. Yes, the tailgate will hold the weight. Worst that might possibly happen is if you slam one of the ramps hard enough with enough weight, you might start to put a little bit of a bow in the tailgate. You can see this on hardcore work trucks going down the road, an ever-so-slight vertical crease next to the handle. This would require more of an impact, not just steady pressure from driving up the ramp.