No Sheep, it's not.
Imagine you have a standard event sheet, which contains collisions, enemies control (create/destroy/etc) and spaceship control.
For every layout (stage), you'll need to include this standard event sheet on the layout's event sheet.
Every layout MUST have at least 1 event sheet that will control spawning on stage. That's what zapmore is saying =D
English is not my native language. Sorry for any mistakes and feel free to correct me if needed.