I'm using a size 5 prince O3 original red racket with it original grip for about 3 or 4 months now. My tennis instructor mentioned an overgrip once or twice after I got this racket. The racket fits perfectly in my hand and the grip never slips due sweat during play. So do I really need an overgrip? I don't want a racket grips that's too big. Also if I should use one, what should I get?

ps. Although I've played tennis for years I've never used an overgrip and I have not expirenced any problems.