To the point
Fuel is regulated by regulations.
Discretionary fuel is at the discretion of the Captain.
More fuel=more options (mostly...)
If company wants you to carry more fuel.... welcome.
I usually put 1.5h more than legally needed. It depends of where you are. if with regulation plus 30 minutes you have 20 places to land may be for you is ok then go for it.
If you are looking for a rule for everything TANK as much as you can to land at just below MLW (make sure you can takeoff is cumbersome to defuel). At the end of the day that is why is DISCRETIONARY and you are the Captain.