I didn't communicate with professors ahead of time and my applications turned out pretty successful.
I think that communicating with faculty would be legitimate if you have special needs that are only going to be met in a small number of departments. Very specific research interests, for example, or if you are applying to tiny graduate programs where it's essential that you like the people there. (My undergraduate college had a grad department with ~8 professors and ~6 graduate students total. Several prospective students visited before they even applied, which makes sense given the size of the program.)
Of course, if you are currently doing research and/or learning cutting-edge material, you can always contact professors to talk about math. I would not go out of my way though to try to talk to people ahead of time "just because". Professors (at selective graduate programs) seem rather annoyed by the flood of applicants emailing them because they think it's good for their chances.
Getting in touch with professors before or during the application process seems more important in disciplines where graduate students are normally funded by the research grants of individual faculty. Luckily, we don't have to deal with this.