Paraformaldehyde shelf-life?

[M]

I was recently told that, when using paraformaldehyde for 
perfusion, the P must be made immediately prior to perfusion.
I have been doing perfusions for years with para made up
to several days prior to the perfusion.  The people who 
have told me this don't seem to have a good reason _why_ you
need to make the stuff up right away.  Does para have a 
short shelf life?  Does "old" para react differently with tissue? 
(In this case, neural tissue?)  A

M