Depends on the protein level of the malts you're using and how clear you want the beer to look when you're done. Whirlfloc tablets help clarify the beer much in the same way as adding irish moss to the beer in the boil. It binds the hazy proteins in the malt to help clear it from chill haze. It's purely an asthetic choice if you use it or not.
IMHO, it isn't necessary to use whirlfloc in all extract beers, rarely needed in BIAB/Steeped beers and occasionally necessary in full mash brews.