What liability? We simply have access to different numbers, but already have the same sensitive information for other purposes. Clients rarely address notices, in my experience at least, and rely on me. Well, when they provide the notices late, it can be more difficult to address. While SC will not send me notices, I can access notices sent to my clients via my agent portal.
I have thought about filing POAs for all clients, but then withdrawing them if a client leaves because I really do not care to receive notices for work unrelated and subsequent to mine. I just submitted a few withdraw requests; state processed same day, I'm sure the IRS has not even looked at them, yet.
At this time, it has been on an as-needed basis. For 2022+, I may simply make it a requirement that every tax client signs POAs if they want me to be their tax preparer and handle tax matters beyond preparation. But, I have some clients where I do not always know what states they need to file (it can change each year), and so I would be filing POAs after tax prep is done unless filing jurisdictions are known in advance.