I tried both methods, and settled on 2.
My problem with 1 clamp was that when you release clamp #1 (such that there are no clamps in the stringbed) the stress is transfered to the 2 mounting points at 12 and 6 oclock.
The tie-downs at 12 and 6 had to be "extra tight" (much more so than the "finger tight" that the manual recommends) -- to prevent racquet movement sideways.
This does not occur if there is always 1 active clamp in the stringbed. Tie-downs at 6 and 12 can be pretty soft -- and there will be no movement of the frame. My machine is LaserFibre 200TT ( it has no outside "crab claws" to prevent sideway movement when pulling the crosses).
Performancewise, I was not able to tell the difference between the two methods.