I am a bit nerdish and believe that any job deserves being done properly. My opinion and I'll stand by it.
I placed my rifle strapped down on a sled that was strapped down a on a weighted table that was tied to the ground. Line bore picture through barrel to target, set on level plain, and scope similar just for a reference starting point.
I used my machinist level, dev lines = 0.0001 of inch, to level the rifle. I used the same level on the scope.
Then I zeroed at 100 yards, stepped it out to 400 yards and set my bottom vertical CRS at 400 yards.
I do not use a level on my scope. I line on target, and when my vertical reticle is drifting centered up and down on POA I will slow and stop my breathing before squeeze. The rifle/scope is level with earth.
At short distance the lack of level is negligible as a miss by an inch is still deadly, but why take a chance? A quick death prevents pain on the animal's part and tracking, guilt on my part.
If you notice that the POI is drifting farther and farther to one side which becomes more severe as the distance becomes greater and greater, it might be a good time to head out and re-level your rifle/scope.
edit; I forgot one thing, sorry. I will hang a plumb bob at distance on a wind free day. I will set my rifle so the vertical retical is lined properly with the yellow string on plumb bob. I will then look thru barrel and ensure the yellow cord is through vertical centreline of barrel, best as my eye can tell. Yes, I overdo things, but I have fun doing it.