Hi Dan,
I have always ground my own meat, and chuck is my favorite for grinding. Actually, there is almost no possibility that e-coli would be on the exterior of a chuck roast. But, if it will make you feel better, wipe it down with a paper towel.
E-coli is begun by unsanitary conditions and spread meat handling and grinding equipment not properly cleaned between batches.
Make sure that your knives, cutting boards, and all parts of the grinder are absolutely clean before use and I like to clean mine in very hot soapy water immediately after use. * A handy tip: wrap a 10-12" long piece of Saran wrap around the grinder exit to keep the juices from squirting wildly around.
Eat your hamburgers rare and relax.
Have fun,
Smoky