When learning a new piece you should play all the way through once. You should play through it again, stopping at all the problematic areas and making note of them, but continuing. After that there are a lot of ways to go about practicing a piece. I think repetitions on the problematic areas in conjunction with working backwards, especially if you want/need to memorize the piece, is fastest.

Playing through the whole song from the beginning over and over again is not an efficient way to learn a new piece of music.