When bushhogging.. i think it is best to use the rpm setting that makes your pto rpm match the recomendation of the mower. in other words.. choose the engine rpm that gives you, for instance, 540 pto rpm.. for a standard pto.. etc. Choose gear or range then to get a good safe ground speed.
For just riding.. I generally use a lower rpm.. what ever makes for a good range/gear combination of ground speed, but keeps the engine from lugging.
Grading is also a lower than mowing rpm.. though higher than joyriding.. enough rpm, in whatever travel gear/range you are using to not lug the engine down when making a windrow.
Plowing.. i pretty much use 'mowing' rpm... you drawbar hp is rated on the engine hp at a hig working rpm. A plow is a high draft implement.. you need the rpms up..
Mind you if you are toting a 1-12" bottom on a 6000# tractor with 100 hp, you could obviously reduce that engine rpm to a lower setting, as there is 'excess' power.. or 'insuficient' load.. etc.
Soundguy