It's interesting that you ask this question because this was addressed in the recent issue of BYO. However, before I get to that here is what I have been doing. I always add Servo + a small amount of additional nutrient to every starter.
Here is what BYO says:
"...yeast nutrients are useful for any brews where the yeast may experience pressure or stress---such as high gravity brews, or if your yeast has been stored for too long, as this can deplete its nutrients. It is also a good idea to add nutrients to yeast starters as they are often started with malt extract."
An earlier part of this mentions that some extracts may lack some of the necessary nutrients yeast need to grow and therefore they recommend adding nutrient to starters. Servomyces is rich in zinc and is a very important enzyme co-factor and also influences yeast flocculation where increased zinc levels increases flocculation after fermentation.
Primary: Belgian Dark Strong
Bottled: Belgian Saison Noel, Chocolate Porter, Raspberry Wheat