is it my imagination or does a rack clamped to the handlebars put no additional weight onto the headset ?
anyway, i think it will take very heavy loads before you are over-stressing things and these will probably be the wheels (spokes) and actual stays on the rack first
i carried a huge load of books to sell to the used bookstore on my front rack and the only problem i had was the rear wheel trying to lift off the ground everytime i took pressure off the seat