I do pay attention, and if I have any doubt about formalin after an
expiration date, a pH test is in order.  With time the formalin can
break down (even buffered!) to formic acid, and the pH is less than
optimal.  

In the case of working with clinical (human tissues) the expiration
date was strictly adhered to, inspectors are less upset with that 
little gig!

Gayle Callis