I have heard that if staples are left too long in 
a cel, they can eventually rust and harm your 
cel. There is also always the chance that if one 
layer is accidently pulled on, it can tear the 
acetate. I always believed it is safer to keep 
layers without staples, just for caution. :) Also 
tape has acid in it, so it may be harmful to your 
cel over time

-Ryouko