There is no simple answer to this complex question. Also, since I am not a lifetime bench presser I will probably get cussed out for this response. So, here goes:
Bench depth is dependant upon several things; performance goal, aestethic goal, past or present shoulder injuries, current shoulder girdle strength, and the passive elasticity of the joint capsule surrounding the glenohumeral joints. If your goal is to improve maximal strength for performance purposes (bench pressing competition), then you must train in the manner that mimics the sport or movement. For hypertrophy of the pectoralis musculature, granted you haven’t had any shoulder injuries, the bar is lowered to a point where there is a slight stretch in the in the insertion point of the pectoralis. If you lie back on a bench and hold a dowel or a stick with no weight, lower your arms as in a bench press motion, you’ll notice the bar will only travel as far as the joint capsule (ligament-tendon construction with several nerve innervations) will allow it to go passively (without additional external resistance forcing a highr joint angle). If you do this, the bar should be around 2-4 inches from the chest. With additional weight the bar may need to be slightly above these measurements to further protect the joint capsules. As far as muscle recruitment, any lower than the passive limit of the the shoulder, the emphasis is placed on the anterior-medial deltoids and the triceps because the pectorals, at their point of insertion along the humerus, will be stretched to the point where they will not have “eccentric control” of resistance. Again, if your goal is to increase bench pressing strength, going beyond that passive point is key. For everything else it doesn’t appear to to require going beyond that limit set by the joint capsule. I hope this helps.