What is the minimum safe voltage for NiMH batteries, if at all?
To avoid permanent damage from a single discharge cycle, a NiMH should not be discharged below 0 v. This should not be done routinely however, as it's likely to accelerate the normal wear and tear on the cell.
For a single cell, this is obviously trivial to achieve.
In a multi-cell pack, it's possible for the remaining cells to take the first cell to run down to a negative voltage, even with a positive output voltage on the pack. This is why most packs or equipment using them will have a warning or shut-off threshold around 1 v per cell, to reduce (but not completely eliminate) this possibility.