I do wait before adding the StressGuard when I use it. If I've added Prime with a WC or dosed the whole tank when I was cycling, I do that in the morning. I only wait until about 5PM to add the StressGuard, cuz otherwise I'll forget. :)
I just pour it in, a little bit in different spots of the tank so there's no chance that it settles in one spot. I try to get most of it in the filter flow so it's quickly dispersed. I don't use it all the time. Only when there's been an injury/illness or when I've added new fish to a tank.
Hope that helps! It really is good stuff. Not necessary for everyday use- plus it would get expensive since you have to use a lot more of it than with Prime.