Take a look at watt v O Grady at light weight .saw a French hw platice I think he was called fight mark potter he did a flying head butt that floored potter .potter got up but was groggy and lost on points.
It probably depends on how successful you can be in making it look 'Accidental' - You may piick up warning for Careless use of the head, But ultimately it might help you win the fight, But of course it flies in the face of Sportsmanship. Some fighters when fatigued may become off balance and ungainly in seeking repeated respitre clinchesl and careless with their heads, but if its found to be intentional than a Disqualifaction should follow, along with Confiscation of Purse, and Period Ban,
A head butter...makes you cognizant of the damage he might do to you...he makes you give up the space you want to be in...he changes the way you attack...he changes the way you stand...he changes the way you defend...and he will also cost you 90 days of healing time...if he cuts you. Armstrong used his head to get under and uproot fighters...Charles would do the same, sometimes controlling bigger men...Andre Ward and B Hop is a modern example IMO.
Ward vs Kessler was a fear the of head fight. Plus Ward switching back and forth from south paw totally confused Kessler.
Also- an inside fighter may have reason to fear a head butter's clash which would cause them to have to fight a style of fight they may not be interested in. Head butting has been pretty effective in controlling the space an otherwise formidable opponent can have in tight places.
https://www.youtube.com/watch?v=KGWrtiB9KD8 If used effectively, headbutts make an effective KO helper.:deal
Good combo by Cotto there, probably the best, clean shots :think I vaguely remember couple of clean KOs set up by a headbutt, but I can't remember, some were caught on replay :think This one was mind boggling [YT]v8y-dRy5NOM[/YT]