This is an interesting question and can go for PVP, PVE and all the hunter’s specs.
For beastmaster mastery will increase the pet’s base damage: the white hits, not special attacks.
If you reforge every master into crit will go with -5, -6% pet dmg.
However the high damage is done with Kill Command and arcane shot who can crit depending on the hunter’s crit chance.
So rather stack crit instead of mastery. Also pets seems to scale with hunter’s crit. They don’t display their crit chance in the stats window, but if you do some tests on a target dummy you will notice that the pet is criting often, the 5% base crit was removed since Wrath of the lich king ended. Back then we had 70%, even 100% crit which was impossible to make it fair for other classes.
So reforge everything into crit and you should aim for 25%-30% crit with pve gear and somehow lower for PVP. Full PVP it should be around 25% crit which is not bad at all. Unsolvable Riddle’s mastery can be also reforged into crit.
Some Marksman hunters are using mastery in PVE, however beastmaster and survival specs are using crit as the second stat after Agility who should be always first.