Whether or not to treat should also depend on time of year and whether you're going to harvest any honey, pollen, royal jelly or anything like that. If you are, then I would only treat in dire circumstances, but if not, then you're free to treat them more often.
Reguardless though, the lower the numbers, the less likely I would be to use chemical treatments, and the more likely I would be to use physical removal methods such as the sugar shake to control their numbers.