Tampico, IL was Reagan's birthplace; Dixon, IL was Reagan's boyhood home. In fact, the recent Reagan stamp had a special Dixon, IL cancel as shown below:

Not sure about the Greenwich, CT cancel, but I think it may have to do with George HW Bush, as he was Vice-President under Reagan and that would have connection to his boyhood home. (GHW Bush was born in Milton, MA but his family moved to Greenwich, CT shortly after his birth, according to his bio on Wiki.)