I use the cheapo cane and/or beet sugar you buy at the grocery store in the baking aisle. Some people claim you can taste the difference between corn and cane sugar, which MAYBE you can if you are talking about a 1-2 pound addition to your fermentables, but for the amounts used for priming it is totally insignificant and doesn't affect flavor IMO.
I always calculate the correct amount of sugar based on the style, volume of beer, and temperature, weigh out the sugar using my scale, then boil up the sugar into a simple syrup solution on the stove. I let that cool a little bit then gently stir it into the beer before packaging. For the relative volumes/weights letting it cool to room temperature isn't really an issue. At most you are going to raise the temp of the beer one or two degrees Fahrenheit by adding a cup or so of solution to a 5 gallon batch.