Tim, the short answer is that it depends. There is more room for debate on the comma-or-no-comma question when we talk about words like "however" at the beginning of a sentence. In those cases, grammar 'traditionalists' will probably argue that you need it while people like me (who believe that language is more about function than rules) would say that you can do without it.
As P.E. Dant correctly pointed out, this is a matter of style. However ;-) I find that putting a comma after "then" and "now" is so rare that it would even be distracting to read it in a narrative.
At the end of the day, the important thing is to be consistent. Wherever you are writing or contributing content, check to see if they use it (and "they" could be your own body of work). From the point of view of non-traditionalists, any style used with intentionality is permissible.