My general understanding is that- If a shop is billing out "book hours" for a job, that is the time billed for, regardless of how long it actually takes.
Keeps a level playing field if on shop says it takes "x" hours another says "y" hours , you chose the "cheapest option" and then you get billed more in the end because it took longer than expected. Also helps authorized repair shops schedule, keep on time and controls expenses on bills and warranty stuff (prevents dawdling on jobs for that extra hour to bill etc).
Fortunately for the consumer, when quoted book hours, they know up front what to expect for a bill. Fortunately for the shop, if the have a really good mechanic, their skill and ability will allow them to do a job in less time. This allows for more jobs to be done in a day/week.
I have never heard of a standard percentage added- that is B.S. Book hours are book hours, if that standard is being followed, that is what should be billed- no more, no less. Can't have the best of both worlds- and that is what it sounds like they want. If they were "allowed" to add 30%time, then that is what should have been quoted in the first place. I highly doubt they refund customers when the job is done under the book hours listed.
In the long run, the shop comes out ahead- which is fine. I think billing book hours is a fair method for operations- if the rules are followed.
I have a buddy who runs an auto shop, he switched over to book hours a while ago. He is now able to give a firm quote on a job- and that is all he bills in the end. The standard he uses specifies to the quarter hour, no rounding up the time.