It's my belief that a spring which is too short is what causes the spring seat to pop out of the perch. It doesn't matter if you paid $300 for a spring that's too short or if you made it too short personally.
There are 6 types of springs. Not all springs have a flattened transition to them. I see nothing wrong with making a flattened transition/flat spot if the original setup had one. I know how to do that, but maybe you don't.
The cars I work on have an even ride height.
If I were to cut the top of a spring, I see no reason to leave angled sharp metal contacting the rubber top hat when I know how to make a smooth finish and I see no reason to destroy or gouge anything when I know how to avoid shoddy workmanship.
When a properly modified spring is lined up the same as the original spring, it should fit into the perch and top hat and requires no more settling than the spring that it replaced.
NUMBER TWO: Cutting a spring will give you a better chance of hitting a bump and popping the spring seat out of the perch.
All Produced springs have a flattened transition on them which lets the spring seat correctly on the shock/strut perch, and the rubber cup of the top hat.
If you cut a spring on the bottom, the flat spot is now gone and the angle of the coils will not hold in their proper indentation......which can create a very big and noisy problem when riding down a rough road, and may abruptly position the whole spring out of it's perch and wedge it at an angle overlapping the shock cup.
This will make the car's ride height uneven and dangerous when steering.
If you cut the top of the spring off, The combination of angled sharp metal and a rubber Top Hat will cause gouges in the rubber and evenly complete destruction, Then you will have a loose spring and much bigger and more expensive problems on your hands.