Those old tillers with those brass straps would flex so much in a breeze.
I generally sail in winds up to 15 knots and, while I am sure there is some flex, I have never noticed it. The length of strap that is unsupported by the either the tiller or the rudder head is quite small, so it doesn't seem to have a lot room to bend. I am sure the older one without the solid rudder head bends more, but even with that one I never noticed a problem.
When I sail, I am always thinking about the rudder load and tiller angle. I focus on keeping the rudder load to a minimum and the tiller straight. If the rudder loads up and/or it stalls I am dragging it sideways through the water, I sheet out and try to figure out what I am doing wrong (e.g. boat heel, sail trim, fore/aft gooseneck adjustment).