Find out how much work your practice has and how efficiently your in-house staff is able to handle it. If the work is manageable, then you, of course, do not need to outsource, but if your employees are burning out, then you certainly should consider outsourcing accounting work.
Employees of an accounting firm often burn out during the tax season, when there is a high volume of workflow than any other time of the year. Consider outsourcing during this time to ensure that the work of all your clients is managed efficiently without any stress for you or your staff.