For me, it depends on which implement attached to the tractor. This is because the tandem axles are offset more to the rear (which is how many tandem axle trailers are built). If the bush hog, PHD, or the disc harrow is attached, I will have to back onto the trailer. When I do it this way, usually the tailwheel on the BH, or the auger bit on the PHD will rest forward of the front wall on the trailer. Also, I usually have to raise the FEL, close the drive on ramp, then gently lower the bucket so it rests on top of the ramp so it will all fit on the trailer. That's a drawback of having a 16ft trailer. I could solve that with an 18ft trailer, but I really don't see a need to upgrade just for that one issue.
If I have the boxblade attached, I will drive it on. My trailer has a 4ft drive on tailgate, and the trailer deck is only about 20" above the ground so my ramp is not steep at all and I'm at a very minimal risk of flipping it over backwards during loading/unloading.
Either way I load it, I get the main weight mass (the tractor) centered just slightly forward of the balance point.
One tip a friend taught me is to measure the distance from your rear bumper to the ground before you hook up the trailer. Once you have the trailer hooked up and fully loaded, re-measure the bumper to ground distance again. Typically you want it to drop about 2-4 inches but that varies depending on if you have a 1/2T or 3/4T truck because they have different suspension rates. For most trucks about 200-400 lbs of tongue weight on the rear hitch is good for a bumper pull trailer, that means the trailer & its load are fairly balanced. For GN's this is irrelevant since the GN hitches are bed mounted and designed to handle lots of weight from the GN hitch.
Here's a picture of my towing setup with the bush hog attached:
PS- Don't worry, there's no trains running on that track!! The switch out on the main line was removed a long time ago!!

I'm parked in an old wood yard where they loaded tree's onto rail cars, but it's now just an vacant 12 acre lot.