There are lots of opinions on this subject. I'll give you my take on the subject.
If you are using a sight that has a seperate adjustment for the vertical portion of the sight you do not need to level to the bowstring or riser. The only important item is that the level is perpendicular to the vertical postion of the sight. This usually occurs using a target type sight.
With most bowhunting sights it is far easier to have the level perpendicular to the riser because there is no adjustment on the vertical component. Many sights do not have individual adjustment for pins in the horizontal plane (left/right).
Generally you can just bolt on the sight and go with it. It is a good idea to check the level against the riser to make sure that it is correct. Depending on the sight you may be able to adjust the level bubble. If not, you are pretty much stuck with what you have.
The third axis is the most troublesome adjustment.
If this is not correct you will have "mysterious" left and right misses shooting uphill and downhill.
The easy way to keep your bow level while shooting on side hills is to overcorrect on the uphill side and then let gravity bring your bow down the hill until it is level. This is of course while using your level