Interesting question, and some companies seem to be better than others
at least for those of us in the animal research area.  However, you
really need to talk to the company tech services if you have doubts.
When working with FITC labelled immunoglobulins for human renal biopsies,
I live within the expiration date, even though the company has indicated
they are good for years at -80C.  I guess this is one difference between
the clinical and research areas, I often go beyond the date in research with
testing and controls.

I've had biotinylated secondaries that last for years past their
expiration date, and also have had them die
on the expiration date, same with strepavidin conjugates.  The result of
this is to ask for the latest expiration dates, if they don't have that, I find
they often will discount a shorter expiration date.  IF they don't, business
goes elsewhere.

Companies have different policies on how they do the dating, and part of
it is they only test to that exp date and not beyond, which means the
antibody may be good far beyond that expiration date.   One company was
relying customers to buy and test beyond the date, and they did not
test on tissues sections in company, but had an extensive staining
manual on how to do IHC using their antibodies.  Putting the cart before the
horse?  Found another vendor on that one.

Gayle Callis