The only problem is many spoke preps go off making building hard. I insist on two different spoke preps depending on the nipple material both of which need finishing fairly quickly or the wheel becomes problematic at tension.
Dry builds are equally problematic in terms of time taken to finish. In fact customer lacing can mean the build takes more time and therefore a higher price than of the wheel builder just laced it themselves.
If a customer brought in a laced wheel but is not fully tensioned, dry spokes or improperly lubricated, not that round or straight dished.... then there is alot of work.
Lacing taking what 15 minutes or less so hardly any money can be saved. Also when i am lacing a wheel i make sure all the spoke nipples start it the same therfore the wheel stays pretty round.
An experienced wheel builder will do the job quicker and right without customer lacing the wheel. and in less time than if the customer laces themselves. I would not accept such a job.
What if the customer pick the wrong length spokes, or laces it with mistakes. This is a mine field. If you want a shop to build the wheel let them and let them calculate the spoke lengths and build it. If you want to lace it your self, build it all your self. The middle way is were disputes can happen.