What is the maximum current allowed through your LED? You must take care of that. What does the 6V mean. Is this the voltage drop of your LED? You're writing about a bulb, is it an LED alone you're trying to drive or a whole circuit with an LED and some other parts (which typically are within a bulb)?
If I have a 6V LED and run power from the 5V pin through the LED to ground would I need a resistor at all because it's a 6V bulb?
What is the point of using the 5V pin when you could one of the I/O pins and set it as an output and HIGH?
On the Ardunio Uno info page it says, "Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board.