matters to me since so much of my 4-cyl. travel seems to be straight up a mountainside... or back down again. If I'm not worried about the clutch and/or RPMs in second gear, I'm fretting about the brakes!
Seriously, though: in my close-to-the-max setup, I fret more about the placement
of the hundred pounds or so of water than I do about its presence. I like the trailer weight
to be as stable as possible, and fluctuations in water level/weight in my behind-the-axle onboard tank had a negative effect on that.
That's one of the reasons I took the tank out in favor of a "gravity-feed" system: Portable 2-gallon containers that are mounted as-needed on the trailer wall, and spares distributed elswhere as the rest of the trip's load demands.
This probably wouldn't matter to most folks with bigger setups, but for my particular tug-tow combo, it does make a difference.