Actually, it always affects the results...however, if the extension length is a very small percentage of the torque wrench length, the result MIGHT be negligible.
The actual torque versus indicated torque is higher by a ratio of the length of the extension divided by the torque wrench length. Thus a 1 inch extension (crowfoot) on a 10 inch wrench will give an actual torque 10% higher than indicated. But a 3 inch extension will give you 30% higher torque!!!!
If the math makes your head hurt, use the following online calculator:
http://www.belknaptools.com/extcalc.asp