What can be legally done, and what is advisable are two different things.
The behavior of a 172/182 at stall will vary significantly with the CG. With two up front, no one in back, stalls tend to be non-events. With someone in the back seat, things can get more lively, particularly if the airplane isn't rigged well. The stall break will be much sharper and there can be a pronounced wing drop at stall. If you're not on your game, the latter situation can easily result in an incipient or full spin. I've been the backseat passenger in this circumstance, and it can be a bit concerning, even when you know the pilot is a good stick. I will no longer be in back, or allow anyone to be in back, during practice stalls.
The behavior of a 172/182 at stall will vary significantly with the CG. With two up front, no one in back, stalls tend to be non-events. With someone in the back seat, things can get more lively, particularly if the airplane isn't rigged well. The stall break will be much sharper and there can be a pronounced wing drop at stall. If you're not on your game, the latter situation can easily result in an incipient or full spin. I've been the backseat passenger in this circumstance, and it can be a bit concerning, even when you know the pilot is a good stick. I will no longer be in back, or allow anyone to be in back, during practice stalls.