You might be interested in the publication:
Carson FL, Martin JH, and Lynn JA: "Formalin Fixation for Electron 
Microscopy: A Re-evaluation"  Am J Clin Path 59:365-373, 1973.

We did a comparison of paraformaldehyde and formaldehyde fixation on surgical 
tissue.  We found that as long as the formaldehyde was buffered appropriately 
(modified Millonig) there was very little difference.  If you use the usual 
preparation of buffered formalin, it has neither physiological pH or 
osmolarity.  The modified Millonig preparation can be used as a dual purpose 
fixative. 

Freida Carson