If by improving the flexibility of a muscle exercised also improves the strength of a muscle thereby protecting the joint, then the bench press is not a very good choice for a bodybuilder in the long term.
the movement is blocked by the chest, widening the grip further restricts the range of motion.
the dip however allows a deeper stretch a greater range of motion.
performed properly, and seldom is the dip is the real deal, the seated version of a dip is the best option for a beginner, as it allows one to use a lot less weight under a much longer TUL, moving slower, allows you to go deep allows you to approach a position that challenges the limits of its ability to stretch at a slow speed, a fast approach triggers a reflex contraction that prohibits a stretch.
a flexible muscle is a strong muscle, I believe this to be a key to a long lifting injury free life.