I have heard the same things over the years but never noticed any "real" difference . If any its a very small amount, for me at least.
I recently went from a ARB which had been on mine since 2005, sold it and mounted up this brush guard to build my own bumper off of. I'm actually getting better mileage now
before and after shots
when I started pulling the old one off so you can see the differences
04 Liberty, Kilimanjaro Edition, Rock Lizard RockRails/Roof Rack/Rear bumper,32 inch Goodyear Duratracs,Moabs,4.10s Matrix 4XGuard front bumper, 4.25 inch JBA Coilover lift and other goodies