I guess it all depends on how much your job is worth. It always amazes me what lengths people will go to for a couple hundred bucks at the risk of much, much more in salary! In my world, my travel is billable. So, not only am I defrauding my company, but my customer as well.
An alternative approach would be to ASK your employer. If they don't have a problem with it then it would be fine. If they are not (or you hide it), chances are you already have your answer.