i used to flex in the mirror all the time when i was bored in highschool. then i got to college and had roomates to it was obviously toolish.
im back to flexing at least one time per day in front the mirror trying to practice poses.
IMO flexing super fucking hard helps to stretch out fascia and increase recovery/blood flow, and increase mobility.
i was getting so tight, but now that im flexing hard and stretching regularly i feel better and not nearly as stiff.