It depends on your icing and your control. Using a smaller bag gives you more control. I use a tip 1 or two fro dots. You can also go without a tip, and just snip the smallest bit from the end of a parchment piping bag.
TOTALLY depends on the look. I also adjust the dot size according to the tier size (bigger dots on the 16" ... smaller ones on the 6"). I've used tips from 2 to 5.
