Is it OK to use an ISO image for SP upgrade?

Hi
The other day, I upgraded from SP3 to SP5.
After mounting the ISO image on another server, I shared it with NFS, mounted it with NFS from the upgrade target, and upgraded it with zipper.

When I read the deployment guide, it said that the ISO image only supports offline upgrades.
Is using an ISO image unsupported even with the method using NFS?