I agree with Dunster also ... SITD will give the design a much more finished look. If you've not done it before, I'd recommend you use a walking foot attachment with a ditching foot for your machine. (What you need really depends on the type of machine you have.) Usually, a SITD foot has a little "blade" in the middle that helps you keep the stitches right in the ditch (the low side of the seam). There have been some threads here about using one, if you want to search around.