If the present participle phrase modifies a noun all the way at the beginning of the sentence and there is another noun between, you should probably use a comma.
EX: I sat down near Jerry, thinking that the ground was dry.
EX: Mrs. D'Amico was in the front yard, trimming plants.
Notice the difference here:
 The mayor took us through the city, explaining all the sites.
 The mayor sold us a guidebook explaining all the sites.