Good point! Mine seems to be quite accurate however. You can check your scanner/printer by scanning a ruler and then measuring the print with that ruler. Check both length and width of the scan. By the way, the paper that plans are printed on can also shrink and expand with changes in humidity and the shrinkage is not nessesarily the same with the grain of the paper as across the grain. Fortunately, the sky does not fall if the model's dimensions are not perfectly exact.
Some things like fusilage sides, I cut and sand to final shape together, temporarily tacked together with 3m-77. It's more important for the two sides of a plane's fusilage to be exactly like each other than it is to be exactly like the plan. (if you want a nice looking plane, ugly planes still fly ok)

Office Depot sells vellum paper in 50 ft long rolls, perfect for tracing off of a blueprint. This paper is nearly transparent when it is layed down on top of a plan. Get a good set of french curves from a drafting suppy store to help you trace smooth curved lines.