If you have decent phones, you can allways throw the mics up in the air and use your ears... I do that quite a bit. Even when you are dfc it doesn't mean you are in the sweetspot. Keep in mind, for most rock-ish shows there is a lot of sound coming off of the stage, and not just the stacks, and on top of that many shows are not in stereo through the PA so dead ahead may give you the best sound wherever you are. If you don't know the room and/or the band you really need to use your ears IMO, even if it is just your ears and no phones. Proximity to the stage, stage volume, mono or stereo pa, volume of the show, sound of the room, the alignment of the stars in relation to Jupiter, type/flavor of mics or pre, your personal taste, and type of music all make a difference in how best to run your rig. To say do it this way every time or do it that way is nonsence. Run your rig how you think it might be best, keep good notes of how you did it, and then try it another way next time and compare. Experience and experimentation are what make good tapes.
my .02
Matt