Ive been running the same spec of motors at 68V for years without issues.

The main thing to watch is that the drivers can cope with this with a margin of safety. Typically you would want 80V drivers to allow for any back EMF from the motors. This is less of an issue with linear power supplies where this is soaked up in the capacitor banks, and applies more to switched mode power supplies ( typically those rectangular metal boxes with terminals on the front are SMPS).

On the current question - I am pretty sure the current just gives holding power at 0 rpm. Higher it is the hotter the motor so I wouldn’t increase it unnecessarily. On many drivers there is a switch to drop to half current at idle to help reduce motor heating.

Motor temps guide seems to be up to 80 deg is OK but cooler is also good. At some point (150?) the magnets can be demagnetised permanently.

Once up and running the voltage is key to performance and to my knowledge wouldn’t help increasing the current setting. Pretty sure it is capped by the driver anyway. Search for the Gecko guide to how steppers work as it is a very good read and includes graphs to illustrate the points made.