Plaids are fabric that must be matched at seams and intersections. Not matching the plaids will distract from the finished appearance. The size of the plaid and the number of times you will be able to use the width of the fabric will guide your decision as to how much extra fabric will be needed to match the plaids.
Plaids require more consideration that a one way design because the vertical and horizontal lines in the plaid will need to be matched.
Some patterns will state that they are not suitable for plaids and one way designs. Watch for this information on the back of the pattern envelope or pattern directions.