Gluing up before planing works well if you have a huge planer you don't mind sharpening frequently. That's normally how things are done in a big shop but in a smaller home shop, you are better off to plane to thickness before glue up, use cauls to keep pieces aligned, then it only takes some light sanding. Planing a glue up can be hard on knives, I'd refuse even you were willing to pay for the sharpening and the set up, not worth the aggravation or wear and tear on my planer.