Stevo is right. Boiling a ton of extract at boil start can also lend some of that extract twang taste, which I try to avoid. You also get slightly better hop utilization with late extract additions.
Whenever I use extract, I add a good portion of it late. But I never use LME, or pitch said late addition at 10-15 minutes left in the boil.
I simply add the necessary amount of Extra Light DME and Corn Sugar at flameout.