Beyond hoping warm water will emulsify some of the fat there's no legitimate reason to think "properly rinsing" beef will affect the fat content to any extent. The fat keeps it together. Considering the quality of most tap water, it might not be the safest thing to do either.
I'll go a step further and say there's very little difference between 80 and 90 % lean ground beef after it's cooked. Some geeks tested it a few years ago.
Also, fannypacks are for fags. Don't let anyone convince you otherwise.