I've never had the extract "twang" but I've definitely had the darkening. I usually add about 1/4 or a third or 1/5 of the extract at start on the basis that my gut instinct is that to boil for the full period of time without any extract at all will have too radical an impact on the hop utilization (I'm too lazy and I'm not fussy enough to actually calculate hop utilization [beer is an *inexact* science]). Although I have done just hops and water for an hour with all extract at flameout with no notable effect. If the recipe has DME, my early addition is all DME because it mixes more easily. I add the rest of the extract at flameout because I don't want to bother with bringing the wort back to a boil. Fear of scorching plays utterly no concern to me as anyone who has ever cooked soup or pasta sauce simply develops the instinct to not to scorch (it called "stirring").
But that's just me. Any method that works will work. If you haven't experienced early addition darkening, or you simply don't care, there's no reason not to do complete early additions.