There have been lots of discussions on the forum about the benefits and drawbacks of using plastic sheets. You shouldn't need to use such sheets unless you're hinging stamps on pages facing one another.
The terms "archival safe and PVC/acid - free" are merely marketing terms meant to capture sales and are not an indication that any testing or verification has occurred.
The issue with sheet protectors is that they are a double edge sword and it all depends upon the environmental conditions.
But enclosing paper, they create their own 'mini environment inside the protector sheet. For example, if the paper becomes damp the sheet protector will keep it damp far longer than if the paper were allowed to 'breathe'. On the other hand, if the external environmental conditions were to become poor for a short period, the sheet protector will indeed protect the paper inside.
All this being said, you will never find any museum or archive that uses sheet protectors and this should speak loudly to us all.
The over-riding and primary factor in good stewardship is to control the entire environment by keeping it stable. Paper 'breathes' 24/7/365 and is constantly trying to normalize with the atmosphere around it, this is why a stable environment is key. Fluctuating atmospheric conditions drives paper to normalize itself quicker and more frequently. A cool, dry and stable storage environment is paramount; ideally temperatures should be held at a constant 70°F with a relative humidity held between 30% and 50%. Don
So I understand environment is very important. Consider factors other than the environment, what would be the best type of storage for sheets of stamps? I have a few sheets in glassine envelopes, but I am uncertain how they are for long-term storage.
I use sheet inexpensive sheet file albums that have glassine storage pockets open on two sides. Quite a few of these albums by Supersafe and Lighthouse are manufactured using poly pages which brings you full circle to the plastics issue. Have never had an issue with glassine.