Are you sure this isn't just a matter of personal preference? I'm finding conflicting grammar rules everywhere and it's really confusing me.
Here is an example:
"Fifth, use commas to set off words and phrases such as however, meanwhile, in fact, in addition, moreover, nevertheless, as a result, thus, therefore, for example, finally and in other words. Usually, place a comma after such expressions when they begin a sentence, and place commas before and after the expressions when they are within a sentence."