I use 25 for speed threshold/parity setting in my slider set. In the NFL, speed matters. It has an enormous effect on how coaches set up their game plans week to week. The only reason I didn't slide mine to 0 is because speed, in my humble opinion, is the most effective trait regarding Madden.
While I wanted that rating to reflect in my slider set, I also didn't want it to be overpowering. After all,the fact that EA has a separate slider for this tells you all you need to know.
Having said all that, if you want to put it at 0 than go for it my friend. Make the game work for you. Let us know how it goes...