I use shoe polish on new tires or if I've changed alignment, or pressure etc. Otherwise I don't. I start on the outer tread edge and do a line down over the shoulder of the sidewall, looking for where it rubs off or stays. I'm middle ground with spraying. I think most people do it just to do it without the technical aspect of knowing if it really helps under the conditions. Since I run Star Specs which are high performance street tires, it would take a lot of runs to need it (probably 5 in a row like a test and tune) so I never end up spraying. Plus the simple fact that I am not that high caliber where the 1/1,000th of a second difference would matter so I concentrate on the driving.