"....That if you have own a big practice, you will inevitably have employees with a few months of heavy downtime."

THAT is a bit of an issue, and always has been in this profession.  If the firm is strictly tax prep, then there are some months with very little for the minions to work on.  Keeping employees busy can be tough.  I've heard of firms that 'force' all vacations are during that down time.  

I'm lucky, MY employees are willing to work really, really hard during tax season and then take it easy during the off times.