It is easy to keep the lines even horizontallly or vertically. the key is in the cutting. Cut along the limes of the plaid rather than using a ruler and rotary cutter. This will take longer. And use cornerstones as Tartan suggests. Finally, be sure that the plaid you buy is 1) woven, not printed and 2) symmetrical rather than asymmetrical. Here's how to tell:
http://www.sewing.org/files/guidelin...ing_plaids.pdf
If you have an asymmetrical plaid, you will need to be very careful in cutting and sewing the sashing, since you can't "interchange" horizontal sashing strips and vertical sashing strips. In short, yes you can, but it will take more time and more fabric (needed to match the plaids). I vote for a bias plaid binding.