Seems like more and more folks are pitching liquid yeasts without starters. I have set up a "answer the yes/no question" template below to help you determine if you should make a starter.
Are you brewing a 5 gallon batch with liquid yeast?
Yes: Make a starter.
No: Don't make a starter.
Are you brewing a beer over 1.060, over 5 gallons, or with expired liquid yeast?
Yes: Make a bigger starter.
No: Make a starter.
For any scenarios not covered here, check Mr. Malty's pitching calculator: http://www.mrmalty.com/calc/calc.html