In my experience, absolutely no problem to cut a coil off a spring. As long as the coil is still captive in the suspension, at full droop, it will not fall out. In fact I bought some lowered springs (name-brand) that were not quite captive, and they never fell out. Only problem was they sat the car too close to the bumpstops.
As long as you are sensible about it, then it is not a problem.
The major problem is when tools cut half the spring away.