As is the case with most betta issues... it depends on the betta. If you have a hyper-agressive male who will flare to the point of exhaustion, shred his tail, and injure his gills, I would suggest doing it minimally, if at all. I have a very agressive male who gets his kicks perfectly well flaring at me. If you have a moderately agressive betta, it probably wouldn't hurt to shake him up a bit with a flare session in short spurts. If you have a shy or fin-biting male, it'll probably send him into a miserable nuerotic frenzy and stress him out too much to be worth it.
I personally do not flare any of my bettas; I do not see the reason. They are all healthy, active, and seem very well entertained because they have an appropriately sized tank which allows them to explore. I change around thier plants and ornaments with every cleaning to keep the environment interesting, and occasionally even move the tanks themselves around. I haven't suffered any lethargic fish yet, so I'm guessing that for bettas, there can be more to life than just flaring. But, if you can't afford a large tank, or prefer to keep your bettas displayed in an environment with few or no plants/ornamentation, it'd probably good to give them some exersize with a brief flaring session.