filling rear tires gives ballast. but it can help lower the center of gravity of the tractor as well. so less chance of tipping over on its side.
when liquid is in the tires. the actual weight is not transferred to tractor and then to ground, but directly to the ground through the rear wheels themselves.
on a different note: i have from time to time been known to move the backhoe so it is on the upper side of the hill. and extended out. to help deal with ugly hills i have needed to go across.
and i have more times "backed" up a hill than what i prefer todo. part of it is because backhoe when in transport position raises center of gravity place the weight on the rear. and i am afraid on some hills i might flip backwards. so i turn around and go up the hills backwards. with mention above of filling rear tires. again helps lower center of gravity of the tractor.
another thing to think about. is tetter tottering effect. when you load the FEL up with heavy material rock to wet dirt. your rear end will become light weight. if you go down dirt road, or over rock drive way. i end up causing mini speed bumps. the rear end will bounce up and down and act like a large dirt compactor. when i load the backhoe bucket up with dirt, and FEL up with dirt. there is less tetter tottoring effect. and less clean up of the dirt road and rock driveway here on the farm. filling rear tires, is lower down the list of were cash needs to be spent at moment, and reason i have not filled the rear tires on the 555C TLB.
if you have R4 backhoe tires on, they will slip and slide, in mud. not much can be done about that. and if you do get stuck, the FEL and backhoe should be able to have enough reach in a direction to pull the tractor out of a muddy situation. been there done that to many times.