Don't head straight for the phones, leave them headphones alone!
Headphones are a massively useful tool for any engineer. They are so useful:
- Listening to that one dodgy musician during a band performance
- To mute the channel and listen to the musician telling you what they want you to change on PFL
- Cut out that one annoying frequency with the EQ
- Provide IEMs
but it's not all good news.
I know of engineers who won't even soundcheck (!) and will just leave the band to it and EQ on the phones (or 'cans'). Why is this bad? You're not getting an accurate representation of the sound coming out of FOH. During soundcheck you should put the headphones down and listen to the way you're changing the sound coming out of the front speakers.
This is because the headphones you're using and the speakers on FOH will sound inherently different when you play the same sound through them - the speakers are subject to natural reverb from the venue or ambient sound and their frequency response, whereas the cans are right up to your ears and block out lots of ambient sound (if you have a closed back pair).
Another reason to not reach for them straight away is the bass response. With a powerful set of subs in place your whole body will feel the impact, but on headphones you don't get the same impact because it's close to your ear.
Additionally, you've got to consider that there will be some sound bleeding from the band in a mid to small size venue. The clarity of monitors and the way the sound bounces back off walls will also change how it sounds coming to you FOH, so don't reach for those headphones straight away.
Mix the sound over the speakers from in front of the band in the crowd (click there to find out why you should). Ever noticed how different a recorded FOH mix sounds on cans afterward? That's because it wasnt mixed for recording, it was mixed for the speaker setup and venue you are working in - don't mix it over headphones!