From a long term instructor and competitor view (who used an iron sight level in Palma and 1000 yard Perry matches), ok, I've used one, a lot.
Since it was on the bottom of the front sight, it didnt cause me to do more than focus on the front sight a millisecond in the process of fundamentals fundamentals fundamentals bang... 10 @3oclock.
Repeat, X at 3oclock. Adjust for wind, repeat, pinwheel x, pinwheel x,
Oh shit, where did the 8 at 3clock come from. I missed the wind change looking at the level and sights... did it a couple of times.
When I put the level on the side of the scope, sticking out beside the eyepiece, besides breaking it off putting it in the truck, plastic piece of shit......
Then put the level above the scope, I found moving my head away from the scope picture caused more time lag and tiny loss of npa and position, the head movement was enough to break my concentration on fundamentals fundamentals fundamentals bang. I shot worse trying to be sure the rifle was level.
AND I've seen what every instructor has seen walking behind the shooters, bubble off to the side. And instructors have photographed me a half bubble off.
David Tubb, created an offset sight set so he could fit the rifle canted exactly to him. And set the sights to that cant zeroed and won several national championships.
He learned that system and made it work for him.
Pay attention. Tubb made a system work for him that is insane to me.
You can teach yourself to shoot a system. Professional sports people will tell you 3500 to 5000 repetitions to get something in there. I found that true.
Using a level to fit a rifle to you with someone helping you is very valuable. Absolutely. Will vote for that every time.
Me, trying to use a level 1200 and under, distracts me, and I shoot worse. The four levels born in my head have served me well, letting me focus on npa, sight picture, no scope shadow, squeeze, bang, and shoot well enough to stay high master in Perry stuff and crossfire my way out of first place into second in a major world Sniper competition. And stay in the top 5 for right at 10 years.
An add on level was useless to me during those 10 years, the 50-75,000 repetitions overrode looking at the level. I do still have that scope level, back in the package. A survivor of my past. I look at it every time I go to the reloading bench. I'm not mad at it or anything.
In shooting the 375 to a mile and 2500, I dont use a level. Watching the wind and splash is way more important to me at that game. The now 100,000+ repetitions have gotten along well with the four levels in my head, and I'm solidly in the F. Galli class/school on that.
Anybody who uses a level to fit the rifle and identify details to fix, is smart, using a tool to remove a variable that hides in the white noise. Good on those who use a level to get to the point fit and feel are natural so their focus can be directed to the next step the shooter needs to master.
I have watched some of the students come from the mentioned classes to different ranges and seen the disconnect from the known conditions of a week of instruction to an unknown range and shoot horribly, then ask themselves if what they learned at class a, b, and c was right for them.
Sometimes I can help them transition what they learned at different classes to the new different ranges we go to, and find a working comfort zone. The newer guys struggle a bit more until they get the fit, feel, and repetition experience into their comfort zone, and get more into the externals that bite them past 600.
All that comes with time, exposure, experience, and successes to grow on.
If a mounted level can help a shooter build a comfort zone (a correct comfort zone), by all means use it.
If it distracts from fundamentals, it's not your friend.
VR.