Here is a link to a previous discussion with comments from knowledgeable people, some (Dave mainly) who are no longer on this board.

Wow. That other thread is a firma thread I haven't posted on!

There is actually scientific evidence supporting the utility of firma rootstock in places where certain Phytophthora water molds are endemic. Abies firma has the best resistance to these molds, followed by Abies bornmuelleriana (which some consider a subspecies of A. nordmanniana ) and Abies nordmanniana. Many of these studies were carried out by Christmas-tree farmers who are trying to get Fraser firs to grow in lower elevations of the Southeast. Ungrafted Fraser firs die in a heartbeat in the southern Piedmont and coastal plain.

If you have heavy soil that is likely to retain water and/or is poorly draining, I would strongly consider buying firs on firma rootstock if you have that option. However, it's also important to point out that hot and humid weather isn't good for most firs; there are only a handful of commonly available varieties which will tolerate the southeastern summer climate, even if grafted on firma rootstock. Abies koreana and abies pinsapo, for what it's worth, are two of the best-performing firs for me.

Good luck! Sometimes you're only going to know if something is going to work for you by trying it!