A day or two wont hurt, and either method will work fine. Whichever is easiest for you. I've done it both ways and have yet to have cross contamination to other plants. The pollen will fall down onto lower branches, so you may get seeds in more than just the bud you dusted. Either way you choose, just remember to spray all the plants down with water 24-48 hrs after you pollinate. That way you kill off any loose pollen that may be hanging around.