If you're installing a sub-panel in a detached building, you will always need at least one and probably two ground rods.

The cost of an equipment ground conductor is very small. Unless you're into some really cut-throat low bid situation, just run a 4 wire feed & then the installation will be compliant in the future if someone else runs a metal pipe, etc out to the building.

Few things are harder to put up with than the annoyance of a good example.