Square to the string is a generally accepted starting place to set a nocking point. Then you go and tune your bow. Paper tuning is popular although just a place to get close settings. You can fine tune all you want. It depends on how well you want your bow to perform and how much you really want to learn.

When all is said and done and your bow is tuned thearrow may be at 90 degrees to the string and it may not be. Every bow can be a bit different.