1.2v is ‘nominal’ voltage (sort of operating middle).
Full/warm is 1.45v. They won’t hold there long generally dropping to 1.35v within 24 hours even without use.
They’ll hold @ 1.2’ish for a long time.
At 1.0 you should consider them empty, especially when using in a series as one of them will be a weak link.
If you push a PACK too far the weak link goes dead > the device pulls voltage through the dead cell and it may reverse the polarity, effectively killing that cell.
If it’s a soldered pack > you’ve killed the pack.
I’d use 6.2v minimum, maybe even up to 6.5-6.6. Probably won’t be a lot of effective operating difference.
Take a look at AA NIMH discharge curves.