I do burp cloths a LOT, but to make sure they still do their job, I don't usually put fabric on them. I embroider the baby's name and put ribbon along the bottom, but then again I had a pretty spit-uppy baby and needed all the burp cloth I could get!
I HIGHLY, HIGHLY recommend the gerber birdseye weave cloths though!!! There are a lot out on the market that don't work as well due to a tighter, thicker weave. Spit-up just rolls right off of them and doesn't get soaked in. Also, the gerber ones are easy to sew with since they're a perfect weight to sew and to do their job when they're finished.