A bubble level has been present on all my front sights (iron sights) for competitive Across the Course and Long Range shooting since the early 1970's. I have also used a level of my own design on all my scope-equipped precision-use rifles (this level mounts on a barrel having a .750 diameter parallel section at its muzzle).
There are many advantages in using a bubble level in virtually any rifle shooting scenario. The bubble is a constant reference to the status of your shooting position and angular consistency of the rifle itself. Relying only on the crosshairs to ascertain level can be a costly mistake. Even tiny errors in rifle cant cause misses. Also, the use of a bubble level allows one to mount his or her sight at an angle so the rifle itself can be canted to better fit the individual shooter (while the sight appears level). The bubble level ensures consistency on each shot.
I watched for years at the attempts to produce a good scope-mounted level, and had yet to see one that met my standards. The criteria for the product we engineered were that it must be a useable, functionable level (incorporating compactness and light weight) that would allow use of the non-aiming eye for discernment and one that mounted on the scope tube rearward main body instead of on the rifle's Picatinny rail.
I will always prefer the forward-mounted cant indicator system since its use requires nothing other than an infinity stare which you are already doing when looking through your scope. However, when using suppressors or not having a barrel with the necessary mounting provision, the Tubb Precision Bubblevel™ (Scope Mounted Cant Indicator) is the next best choice.
~~MADE IN THE USA