You make a good point Paul. I use a level on the riser, mainframe and a plumb line when scope leveling. I also use a scope level as well as the riser level...and as Paul said, the riser level is usually out. However, it is consistent and I believe closer than the naked eye therefore worth using. Eg, on my Frankenbow the scope level is true but the riser level shows a slight cant to the right side of the bubble and I know I'm level. If you're using a level and find your arrows are drifting low left or right at further distances, try canting your bow slightly left or right and watch your arrows come back to center.
I got a couple of different types of camera levels off eBay. After leveling up the bow with them and a long carpenter level on the limb tips I put scope level on the scope mounts, to match the other bubbles. The scope level is right in my peripheral vision where it needs to be.