I have noticed a bit of tyre slip when first driving off having liberally sprayed tyre dressing on my car tyres, so have never dared try it on bike. I guess I could be a lot more sparing with it, but not a risk I want to take.
Personally, I wouldn't go anywhere near my tyre sidewalls with any form of tyre dressing. Most of the cheap products for cars cause have a huge amount of fling that can travel up the side panels, but even with the non-fling dry touch dressing I use, I still wouldn't put it near bike tyres. If the tyre sidewalls do look like they need a bit of a tidy up, I'll approach them the same way as I do car tyres with an APC and tyre brush. This will get rid of any browning and leave them cleaner, but no back to black for me
There is a very well known dealer on Westgate road, Newcastle upon Tyne. They used to do the whole tyre, sidewall and treads! I assume so it looked a lot newer. Bloody idiots.