Emmanuel tells us of Bouin's/picric:

"..I never use these solutions because it introduces a lot of acid in the final
fixative (phosphate-buffered formalin), which does not have enough buffering
capacity to neutralize it (measure the pH the next morning!). "

Hardly the point. What is the pH in the tissue once it has gone through all the alcohols and xylene and wax, each leaching out some H ions? You don't stain the residual formalin, but the tissue. 
Besides which, it is general experience surely, that unbuffered formalin gives better IPOX results.

Terry L Marshall B.A.(Law), M.B.Ch.B., F.R.C.Path
Consultant Histopathologist
Rotherham General Hospital, Yorkshire
terry.marshall@rgh-tr.trent.nhs.uk