Your sugar probably isn't going to mixed well. You may a have a few bottles that don't carb well.
It's best to dissolve the sugar into warm water and let it cool. Then put that into the bucket and siphon on top. I know you knew that, but if you forget you can always add the water sugar to the beer and gently stir it in. It's no biggie so long as you don't splash it around while stirring.