It used to be that I would put the zip code in ePrescribe as my first filter....Now, the fax number and phone number take up that spot.
I think the default (when no preferred pharmacy is entered) is for it to bring up a list of pharmacies based on the patient's home zip.
So you should not need to put in the zip code at all.
We have patients include their pharmacy as part of their registration demographics so when I see them, the front desk staff has already entered it.