Larger / longer / wider tractors are more forgiving of not filling the rears. When I first got my DS4510HS, the tires were not filled. I used it for a while and It was much more stable than the LB1914 on any task even though the LB1914 had filled rears. But the larger tractor was substantially longer, wider and heavier.
One day I was lifting a heaped bucket of wet dirt (probably ~1,500 pounds). The tractor was pointed down hill when I raised the bucket to dump it. When the bucket got about 7 feet in the air a rear started to lift. So I lowered the bucket. Dumped it and made an appointment to get the rears filled with ~1,000 pounds of whatever the local JD dealer was using (not calcium).
One day I was digging a stump out with my filled rears and a 500 pound blade off the 3pt. I got the bucket hooked under a root. Curled the bucket to try to break the root and the entire rear of the tractor came completely off the ground. I also bent the bucket. So now I have filled rears and an additional 350 pounds of weights attached to the end of the draw bar a few inches inside of the 3pt eyes so I can leave it there all the time and not interfere with implements or even more ballast. Now when I know I'm going to go really heavy on a lift, I put an extra bucket on the back with my 3pt to SSAQ adapter and fill it with heavy stuff (like extra bags of cement that were left out and got hard). With filled tires, 350 pounds on the draw bar and 1000+ on the 3pt. the rear stays down.
Moral of the story - even if it doesn't seem that you need filled tires now, eventually you will do something to make you wish you had them. And in some cases you will want both filled rears and big ballast on the 3pt.
The only down sides are that the tractor is heavier for times when you want it light. But if you only fill the rears, filled rears still don't tend to sink as much as unfilled fronts. The front is what tends to dig in. And if you have R4 tires, they have so much contact patch that I don't ever see it being an issue. The other down side is that you won't be able to go as fast up hill in the highest gear due to the extra weight - kind of an obvious one.
