Advice on these steppers -Nema 24 Stepper Motor 24HS34-3008D

[m_c]

As long as you set the current limit on the drivers correctly, it didn't ought to be a problem. OK the current will rise quicker, but the peak current should be the same as it would be with a lower voltage, it'll just happen quicker.

The big issue is as the motor spins faster, the back emf increases (emf = Electromotive Force, essentially voltage). Back emf is why you need a higher voltage supply, as in order to over come the emf and 'force' enough current through the windings, you need a higher voltage.
A modern drive only controls current which in turn limits voltage. The voltage over a motor coil, is related to the back emf being produced (higher inductance = higher back emf), with the highest voltage being the maximum supply voltage. As back emf reaches the point at which the supply voltage is unable to fully overcome the back emf, the coil current starts to decrease (and torque, as motor torque is proportional to current)

Now the problem comes when you have too much speed, and too much voltage.
Take the motors in question, with their recommended 57V, and 4.2A current. At a high enough speed to use the full 57V, the motor is using around 240W. Bump that to 70V, and power jumps to 294W, about a 20% jump.
Now can the motor dissipate that much heat?
(not all that power ends up as heat, but steppers are not that efficient)

It's where you've got to judge where you want to compromise.
If you never run the motors that fast, a higher voltage supply is not likely to be an issue.
You could reduce motor heating by reducing the current, but then you loose torque.

Ultimately, the best measure is how quickly the motors heat up in use.
If they get too hot to touch within a couple minutes of running, you've probably already cooked and demagnetised the rotor, and now have a poorly performing paperweight. Rotor overheating is what will kill a stepper motor, and as it's got no direct cooling, you're relying on heat transfer to the motor body to cool it, and that heat transfer is pretty poor.
If however after an hours running, they reach and plateau around 60-80deg C, you've probably got them on their ideal limit.