Going into the hive always sets them back to a certain extent each and every time.
But, who cares? If you are going for a record honey crop, or maximizing queen production, or wanting to achieve some other production goal, then, yes, you have to carefully time and minimize when you mess with them. But if you have bees for fun and want to learn, then go in whenever you want and weather conditions allow.
I would also say there is no set best frequency for going into the hives. Different times of the season require different amounts of inspections - more often during swarm season when monitoring hive conditions if you are trying to prevent swarming, less often (if at all) during winter when not much is happening inside the hive. Also, how deep you go into a hive influences how disruptive the inspection was and thus how often you can go in with little negative affect.
JMHO