A couple points. The main use of belt dressing these days is for the smooth side of the belt. As the belt ages and becomes harder, it'll get slippery. When it run against a tensioner pulley or an idler pulley it can then sometime slip, creating a squeeling noise. The belt dressing will help alieviate that noise.
Certainly make sure you have the best belt you can get with the proper length that makes it able to get on, but isn't soo loose that you don't have to work at it a tad.
The latest thing I've heard folks doing to help with belt slip when they have the proper belt is getting the surface of the pulley more agressive. usually through sand blasting it. That removes the smooth surface from it with the theory it'll give it more grip. I'm not sure if I buy that myself, as I think if you have a slipping issue, expect to eat some belts pretty rapidly if you sand blast the pullies. But it may be worth trying at the track. I take my theory on this from industry, where I've yet to see a pulley that isn't smooth running any kind of V-belt. They just add more belts, more pullies. Or they go with a cogged belt.