I think I originally heard this from Tommy Emmanual as well as Glen Fricker @ Spectre Sound Studios, but make sure you tune your before you play.
This helps for two reasons:
Your guitar sounds better
It inherently helps with your ear training so that you can know when your guitar is in tune and work out what notes are used in a song quicker.
It also helps work out what strings seem to be an issue on a given guitar. My low E string tends to go out of tune before the other do. Also note that for accoustic players, your guitar will go out of tune a lot depending on the weather or humidity in the air.