Its just as safe as any spring thats shorter than standard. Cutting a coil off wont see any issues other than the car bieng closer to the ground. Simple as that. The spring will not fall out of the cups, you will not pirouette off the road in a fireball because you've cut your springs down. It is perfectly safe
I'm talking from exprerience here BTW, i've driven a car with cut spring 20 miles just this morning, just as i've been doing every day for a year.

I'm not trying to cause argument here or anything, its an interesting discussion that i've seen on many forums and it always produces some conflict, but I reckon thats because a lot of people say CUT SPRINGS = BAD without knowing much about the subject.