At some point you will need more yarn to do the neck. You can look ahead at your instructions, if you like. They'll probably have you start the neck from where you finish the front of the sweater, using that same yarn. In that case just cut the yarn for the back, leaving enough of a tail to sew a shoulder seam if you like. Really, I wouldn't worry about it too much. If you need to join in new yarn, especially at a seam, it'll be completely invisible and easy to join.
In any case, you'll need to cut the yarn at some point, so you can start the sweater front! LOL.