Dosing by a subjective, non-verifiable number is stupid. By that practice, a nurse runs the real risk of not medicating appropriately.
I mean there are some who will underreport pain (my pharm prof would say "the bachelor Norwegian farmer type") -- they are determined to remain stoic. So POD#1 after a double knee replacement, pt is gritting his teeth, grimacing, and PT will be here in an hour. Is plain Tylenol appropriate vs an opioid -- ONLY because the pt rated their pain a 2? I've had pts do this but agree to meds after some education, e.g. "you look miserable, and you have PT in an hour. PT can be painful. I'm concerned that without treating your pain, your PT will be less effective -- if you can even do what the PT recommends. If you can't do PT, your healing will be significantly slower."
Conversely, there is the type who if conscious will always rate their pain an 11... is it appropriate to give a mg of Dilaudid when the pt's resp rate is 6 -- ONLY because the pt rated their pain an 11?
If pt care doesn't require critical thinking, if it's appropriate to mindlessly "follow orders," why have licensed nurses?
If my nursing judgment says the order is inappropriate -- say in the 1st example -- and the MD refused to write a reasonable order, I would probably suggest the pt that the opioid can't be given unless they rate it >5.