I am not worried about perspective in most of my collages. A smaller item may be superimposed over a larger piece of ephemera. It never bothers me in collage, but it might bother me if I was doing a landscape collage for instance. I really never gave it much thought though. I am more worried about balance and color and total composition. I like oddities and figuring out how to make those oddities work. Somehow my eye just knows what works and what does not.