for long term perspective it is better to keep each banknote separately & preferably try to use mylar, I have seen few older proof/unc sets which come in plastic and has degraded even the coins, ex 1969 india proof set, 1947-53 unc set pakistan. There are few plastic sheets which has acid and chemicals can damage notes, you can get a sharp smell out of those plastic sheets. I bought few in India and then just replaced with mylar..
Banknotes are just paper, after lot of research I'm now using following mylar sheets:
http://www.oregon-pioneer.com/gpage2.htmlI posted the same on a similar thread in this forum.
go for Mylar 5 x 8, even our hyderabad 100 rupee fits in it perfectly and still leaves some room..
I have seen these with many dealers in US.