Just to reiterate, that is a personal choice. I never make assumptions when dealing with someone's safety, I can only provide you with my thoughts based on what I know. I run several thousand watts in lighting and haven't had problems. That isn't to say they don't bust in the door tomorrow, that is a constant threat we all deal with daily.
I will say that 600W is easy justified as a small AC unit or space heater depending on the time of year and I do not believe anyone would ever look twice at that small of an increase in usage. My rates at ~.09/kW would increase total costs by $29.16 running 600W at 18/6 for 30 days.
It is your call to make.