At what velocity of a stepper motor do you need acceleration profiles?

There is no simple answer to that. It depends on the load on the motor and the torque of the motor relative to the torque needed for moving the load.

I think the only reliable solution is trial and error with the actual application.

I guess the safe solution is to use acceleration unless there is some reason not to.

I have been wondering, but have not got around to testing, if a step-wise acceleration would be sufficient - for example get to 1000 steps/second by first running at 350 and then 700 and then 1000.

Of course you also need to consider whether the application can accept rough treatment.

...R