The reason not so many people use the string method is because it lines the wheels and not the chain. If everything is where it should be, that method will work, however it will also give you a false alignment of the chain itself if something is out of whack. If you sight down the chain, you are aligning the chain with the motor, rear sprocket to drive sprocket.
Using the string method can result in problems if you have a tweaked wheel or frame, usually a dirt bike problem.
If your chain is arrow straight, you should be good to go, but if you use the string method and sight down the chain and it is cocked to one side or the other, you've got bigger problems.
Anyone born before 1970 has probably heard of and used at some time, the string or two by fours or etc... method.