There's no hard and fast rule to wall safe placement. To start, I suggest you consider what you're immediately going to put in it and determine in what room those things are usually going to be initiated? Most wall safes without a mechanically attached concealment (mirror, artwork, furniture, etc..) are usually behind a 'plain site' concealment (an open door, clothes on a clothes rod, an easily movable object, an out-of-the-way spot, etc...) There really is no commonplace for the modern wall safe; they're designed to insert in most any wall so as to reduce remodeling costs, and facilitate regular use.
The primary defense against thievery is that the wall safe is never detected; on the other hand, a lot of people are more interested in convenience and the wall safe is there to slow down or deter misuse of trust and/or objects (prescriptions, keys, weapons, cash, etc...).
At Safe and Secure Co., we allow for all those considerations. All the safes are on the truck -you make your selection(s) in your driveway. All the hardware, for most any type of concealment (hinges, slides/glides, magnets, etc...) is on the truck and a good concealment object is probably already in your home, if necessary. Our specialty is determining a location that is agreeable/accessible and our construction experience streamlines the installation process for any given situation.