All of the spray on tire "cleaners" contain silicone or another agent that reduces cohesion to lift the dirt. Remember cohesion is the same as sticky when it comes to tire grip and the road. There is no way in hell you're going to get that stuff on a tire and not get it on the tread. Reading this post makes me wonder how many riders have thought their tires were crap or getting hard when in fact they had just "greased" their tires to act like ball bearings!
If you use these products then spray if on a rag first and then apply it. Be sure to stay far away from even the edge of the tires tread. The last thing you want to find out is that the nice shine of your tires just put you and your bike sliding out through a curve, this goes double if you are a knee dragger. My advise is don't do it, if they are that bad looking they're probably dry and hard anyway, replace them, it's cheaper than skin grafts.