I have an interesting story on the subject. When I worked for my dad, all of our clients were in-house. Nothing we appraised was sold to the secondary market. We did however use a FNMA form (2055, 1004) when providing appraisals to our lender clients.
Eventually it was time for me and another guy that worked for my dad to interview for our licenses. Our interviews were probably a month apart. We were both chewed up one side and down the other by two senior board members and the investigator for using FNMA forms for non-secondary market lending purposes.
Their reasoning was, because the forms were FNMA forms and stated as much, the client and other readers could reasonably assume that the appraisal was FNMA compliant because it was developed on a FNMA form.
So with the boards threat to drop the hammer if we didn't change our evil ways, we switched to GP or AI forms for our in-house lenders. Every now and then I'll get a client that wants an in-house appraisal performed on a 1004, so I retell this little story.
The short version is: If it's an in-house lender; I only use GP or AI forms.