I think it depends more on where you locate the straw in relation to how you hold the cig. I use a 901 and the straw rests on an upper side of the cart when smoking. When you stop pulling/drawing on the cig the liquid wicks up to the atomizer and air comes back down the straw (gravity will keep the liquid away from the straw when it is held horizonally, and this is more apparent as the cart empties).
If the straw was on the bottom I could see it getting flooded with
juice and not doing anything, but since mine never are I haven't tested that. I can tell you that my wick is almost 100% dry when I change the cart, and I get full vapor for the entire time I smoke it.
The straw seems to work best if it is protuding just slightly past the wick, but again this is only my finding. I do know I can get up to 10 drops into a 901 cart with a straw and loosely packed poly fill.
I don't think the straw does anything at all when you are drawing on the cig since there is equal vacuum across the entire end of the cart, so no air could get back down to the bottom of the cart until you stop heaving on it.