What do you think about warm-uping the posterior chain before every work-outs?

Lately the first thing I do when I'm ready to train is a set of 20 reps (very far from faillure) of bodyweighted back extension followed by 2 sets of stretching because I lack flexibility in this area.

Is it a good idea? Could be a suggestion to others too if it's a good idea.

Staying injury free is one of my important goals.