The seated behind the neck barbell military press is an exercise that has fallen out of favour in recent years. It is typically associated with shoulder injury and impingement. However a healthy shoulder joint should be able to press behind the neck and I often use it as a diagnostic tool with a broomstick during my initial assessment of a client when they first come through the door, something that I teach personal trainers in our CHFI Performance PT Level 1 Certification.
Having strength throughout the entire range of muscle group not only ensures full development but also protects against injury. Lack of mobility or motor control in the extreme ranges of motion will likely lead to joint pain and dysfunction predisposing you to higher chance of injury.
If your shoulder mobility is not up to par I suggest addressing that first. If you have good shoulder mobility then I recommend about 50% of your shoulder work should be done behind the neck to maintain structural balance.
I prefer to do this exercise seated and without back support as demonstrated below which increases the range of motion, thus allowing you to build more functionally sound shoulders that not only look the part but are strong as well!