I'd say no, under most circumstances dont masturbate too soon before a fight. Maybe leave it an entire week, that would be enough to build some useful sexual frustration to take out on an opponent. Your bound to feel sharper and more aggressive if you havn't ejaculated too soon before. Lets be honest how motivated does anyone feel afterwards?
The experts will say that regular masturbation/sex keeps testosterone high and that to drop off for a significant number of days/weeks has no benefit to a fighter whatsoever! Fighters in the past would abstain for an entire training camp. Muhammad Ali used to......and spoke of 'Mother Natures' wet dreams! Abstinance of anything promotes discipline and helps a fighter focus whether it be junk food, alcohol, drugs, cigs or sex!
Others will say that it takes the edge off a tense fighter and is of benefit. What works for someone may not work for another.