We always use string-lines when we are building a straight wall. Always. We tell our students to use them and teach them how to use them. To think that people can build straight without them is pretty crazy. We occasionally, to get round something or access a tricky spot may remove a section temporarily or we may untie them to move a lot of stone across to the other side of the wall, but essentially these lines have to be there while we are building and they are in fact essential in the process of building a good straight wall.
And you can't just put them up and ignore them. A student has to know what he's looking for. He has to be able to line up his stones according to the level and plane that these string-lines are tracing. You have to check and keep checking them. And they can't be set up wrong or attached to loose batter frames or aligned incorrectly.
We have two ways of putting up lines. They can both be very useful. The first requires putting up one line either side of the wall, up to each level you are working to. When this level is completed the lines are raised to the next level. This is very good for doing walls with all the same heights of coursing. It makes it very easy. It helps too if you are creating, or in some cases recreating (repairing) a random coursed wall. However it does make it difficult to have several students working on the wall at the same time. There is inevitably always someone inadvertently leaning into or pulling back the guide lines while someone else is trying to sight down them.
Sometimes the solution to this problem, especially if you are building in a more random coursed style or working with more irregular shaped material, is the second method of setting up string lines – a method that I learned from Patrick McAfee a terrific walling instructor, heritage masonry author and stone and lime mortar expert from Ireland.
You set up two pairs of lines well above the height everyone is working. This way you can sight down onto the two lines, line them up and maintain the plane and batter of the wall with each stone you place with its face aligned along the same plane defined by these lines. All the while people can still be leaning over the wall or moving stones and not disturbing your sighting. Closing one eye and taking care to fit each stone properly you can keep all the stones faces flush with the wall along three dimensions.
It is still important to step back occasionally to ensure that the stones are all running level and horizontally along each section of random coursing and that you are still covering your joints all the time.