If you're reading a low E, with three leger lines, you have no choice but to play an open string. An edited piece of guitar music (classical guitar music, that is) will often include numerals with circles around them. These refer to strings (1 for the high E-string, 2 for the B-string, etc). Where there are no circled numerals, you'll just have to decide for yourself what fingering works best (or consult your teacher).
