Mice aren't really too big of a problem. They won't kill chickens or eat eggs...Rats on the other hand can be a huge problem. I had a problem about a month ago. i was putting out 8 bait block a day. Fianally now they are gone but I still keep bait blocks out to avoid having more hang around!
I'm not worried about mice killing them to eat them or anything. Just that they will eat their feed and spread disease and such. I'm not paying to feed the neighborhood mice. =P