Don't serve your cocktails too cold, but don't serve them to warm either. You want to hit some target comfortable temperatures that will give you the right dilution as well as the right chill fact for your drink.
Ice plays a big role, dilution plays a big role when used correctly and temperature can change both the aroma, mouth feel and flavor of your drink. Too cold, and you'll be serving out a bland drink. Meh!
The best temperatures for your drink:
Shaken Cocktails: 14F to 23F (-10C to -5C)
Stirred Cocktails: 19F to 32F (-7 C to 0C)
Brandy/Whiskey: 53F to 62F (12C to 17C)
Champagne/Sparkling Wine: 42F (6C)
Straight Shots: 60F-64F ( 16C to 18C)