We used to have  this very problem and were told the same thing about 
formalin fumes affecting the  quality of the Wright's stain in bone marrow 
smears.  It is true.  We made sure they were kept them away from formalin, 
and the problem diappeared.  I don't know the theory behind it.  I believe 
there is an article in a book we have at work about troubleshooting special 
stains, but I'm at home and can't get to it till Monday.  I'll look it up on 
Monday.

Ellen Yee
Central Histology Facility
Sacramento, Ca