My question is how do i ensure Cass actually drinks whiskey: do I simply put it into her inventory and ensure she never runs out?

Could it be that east?

The perk gives YOU a DT bonus when you drink whiskey. Putting the stuff in her inventory doesn't do anything.

F.Y.I. Cass' perk scales with the Survival because whiskey is a consumable.

At 100 survival you get +3 STR +3 CHA +6 DT. You can do alot with this perk if your committed to stockpiling whiskey, which isn't hard and is moderately pricey.

7~20 cap is pricey?

ahem. moderately pricey. A little cheaper than most chems that offer similar effects, so I say moderate.