I don't image" healthy drives, but I do backup all personal folders on any drive (not onsite), before any service work.
Any "failing" drives get imaged and stored for 7 days only. If my customer doesn't call within 7 days the backed up data is deleted.
I usually have about 4 Terabytes stored at any one time.
All procedures are clearly stated in my "Terms Of Service" and must be agreed to before any work is carried out!
Also, it is explained to the customer that data loss as a result of any service work is NOT our fault.
We can not be responsible if customers are too lazy to backup important files!
(I have had drives that appear healthy suddenly expire!)