I tend to slightly under-pitch the yeast in my beers. I use the calculator in BeerSmith as a guide to my starter size with liquid yeast and get within a few billion cells of what they recommend. (There are other ones out there that are probably better, like Mr. Malty or the Lallimand one that Brewbama uses). For example, if the calcultor says 3.2 liters, I make a 3 liter starter usually rounding up or down to the nearest 0.5 liters. I always get good results and making the yeast work just a bit harder doesn't seem to make any difference in flavor to my palate.
With dry yeast, I will pitch one pack (I do not re-hydrate since it is no longer recommended by the manufacturer) in a beer up to 18 degrees Plato or so, and two packs in a big beer (20+ Plato) like my RIS.
Just my .02.