Should I primer my walls/ceiling before I spray on texture?
You should.

Something is needed to seal the new drywall.  Primer is much cheaper than paint.  The basic rule is if you want a wall to look uniform, you have to prime it first.

If this is a paint + texture can, when left unprimed, the drywall will suck all the paint from the texture into the wall, leaving an uneven mess behind.    If you're going to paint after applying the texture, I'd prime in between as well; the natural unevenness from the spray will lead to splotchiness.