I have two wheel weights per side, plus calcium (yeah I know the downsides) in both rears on my 5420. I still need my 1000 pound back blade when I am working in the dirt. Without the back blade, it is not real stable. With the back blade I feel comfortable working on my uneven terrain. I have the 541 MSL FEL. Even with the back blade on I can still pick up the back tires, but it is not easy. Note that you can add as many as 4 weights per side, but then they stick out a long way beyond the tires. Begging to catch on something. Also difficult to remove when not needed...
All weights are about $1/pound from JD. A little cheaper in the aftermarket.