I use a sight level all on my bows, however is is sensless to have one if your sight does not have thrid axis leveling. If it is not checked the bubble might not be level with the string. I have however set the level on the sights without the leveling capabilities by shimming where needed until the sight bubble was level. This can be done with a torpedo level on the string. To do it properly you need a bow vise to hold your bow upright to set the level, then drop the bow vise down to make sure that the bubble stays the same when the bow is tilted down. One can crudley set the level by holding the bow and checking it with a torpedo level and making adjustments as needed until the levels match. The bubble level with help greatly with angle shots downhill. The ultimate way to set the level is at full draw in a hooter shooter. Many of the bows today will have a different string angle when the bow is at full draw.