No, you don't have to consider yourself a witch to practice the arts. The only thing you need to know is how to direct it. Technically, a witch is one that practices the arts and everything from divination to spell casting and summoning can fall in the category, so by definition one would be called a witch if one cast a spell.
If you want but as was already mentioned no you don`t have to call yourself a witch you may as well call yourself a priestess or whatever.
Only thing you need to be for spells to work is dedicated and to have at least basic knowledge of what and how you want to do as well as some skill in energy manipulation.