Is a yeast starter really necessary?
Before pitching the yeast into the wort I always consider if I need a yeast starter, well when it is time to pitch the yeast it is far too late really! A yeast starter needs time, a day or more! A yeast starter is used when the need for more active yeast cells arises, for instance when fermenting a big beer like an Imperial Stout or a double IPA. If you pitch too few active yeast cells it can lead to the fermentation not starting, going much slower than normal or giving a different taste profile than desired.
When using dry yeast there is enough yeast cells in one packet for 20l/5 gallons of strong wort and if you have more wort than that you can always use another packet.
If you have a "smack-pack" from Wyeast the nutrients for the yeast is included, you will just smack it in time for it to be ready for pitching.
But if you reuse yeast from an earlier batch of beer or if you use a slanted yeast tube you will probably need to do a starter to get enough active yeast cells for pitching.