Speaker height vs ceiling height

I would think you want to avoid early reflections of ceiling similar to walls. Usually there is less distance to work with in regards to ceiling and floor than walls even.

I like to keep my speakers as low to the ground as possible but also with adequate elevation from floor. Having drivers as close to listening position elevation or slightly below is probably a good practice in general.

Tall line arrays might be a different story, but good ones that tall will cost a fortune.