I think it depends on whose stocking it is. If it's someone close like a sister and you know she likes this type of thing I think it's fine. Putting them in a stocking isn't the same as wrapping them individually and giving as a gift.
I like to include unused samples alongside a christmas/birthday present to my family and friends... I personally don't think it's tacky, however I do think it is if that was the ONLY gift you were to give someone (but each to their own).
For a *stocking* Not tacky at all.
As an actual gift-gift, tacky. Although, saying that, Many of the Sephora gift sets that people buy as regular gifts are nothing but a box of deluxe sample sized products. So... I guess presentation plays a role in that too. (I bought a lot of those gift sets specifically to break down for stocking stuffers.)
My family does stocking gifts (small, practical and low-value type items) for Christmas eve and Gifts on Christmas day. So I do set aside samples that I won't use and give them away in package exchanges or as extras in a birthday parcel or as stocking stuffers, depending on the individual. I wouldn't give sample sized cosmetics to someone that doesn't wear makeup, for example.