Ascorbate and HEPES

[Przemko]

Hi!
I have a question concerning tissue culture.
First of all bicarb buffer that is so commonly used is in fact 
quite lousy (as opposed to HEPES) at physiological pH. So, why
do we still use bicarb and not HEPES. I know that some people use
but it appears to be not a SOP.
Secondly, ascorbate is known to be rather unstable. In fact it 
appears that, in solution, it will be good for a day or two. On
the other hand medium such as alpha MEM contains ascorbate. Companies
guarantee this medium for a year or so which is fine exept that if I 
would like to add ascorbate (because I work on e.g. collagen) then
the problem begins. Do I add? How much? Will the decomposition product
screw me up? I asked various technical services at various companies
but NOBODY gave me a decent answer (well that is how we do it...).
Any pointers?
ThankX
Przemko