If there are dispensers in the bathroom then, yes, I'll have a small sign that tells the guests what it is, ie.
"Dispenser use in this guestroom may save up to 1,000 plastic bottles from entering our landfills each year. Shampoo is by X, Conditioner is X, Shower Gel is X"
With a nice font on parchment paper and put in a small frame, it tastefully lets guests know what they're using and why.
If septic systems are an issue (and they usually are in the countryside where I tend to work), I'll usually have a small sign on the toilet that says
"Please be gentle with your use of water and ever mindful that our small septic system works best with only the very smallest amount of tissue"
Again, being done tastefully is key (I hate finding laminated notes tacked to the wall with rules of do this/don't do that).
I also put information about all of the inn's green initiatives in the welcome books.