This answer appears on "The Grammarphobia Blog." It argues that the relationship between the comma and too is a stylistic, rather than a purely grammatical, one.
<<A comma, too?
A: In the universe of yeses and noes, this is a maybe [referring to a question about whether to use a comma before too] (which explains why two intelligent people can disagree about it). There's no grammatical rule that says you must use a comma with "too" in the kind of sentence you describe. It's largely optional, and depends on the inflection the writer intends. In the case of "too," use a comma if you intend to emphasize a pause.
Take your example: "Steve likes chocolate ice cream too." Context might call for a comma or it might not. If Grandma has just given Steve's pushy little brother Sam a scoop of ice cream, and their mother wants to suggest that shy little Steve should get the same, she might say, "Steve likes chocolate ice cream, too." (With a little lilt at the end, emphasizing the "too.")
But if Mom is just describing a catalog of the stuff that Steve likes, and has already mentioned, say, vanilla ice cream, she might say, "Steve likes chocolate ice cream too." (No particular inflection there.) It's often a judgment call.">>