Be nice to the engineer, we want you to sound as good as you can. A sh*t sound drives the punters away.
My basic rule of thumb (which goes for all bands on a mutil bill or a single bill), always take from the post-di if available, keep the PA as flat as possible and just compensate for the room. Your sound FOH will always be different once you have a room full of people, and will be compensated as such to get you as close to "your" sound (band and instrument) as possible. Know your gear, the amount of times I've asked about post-DI to get a blank look.
I will always do the best I can with the time available, remember it's a sound/line check not a rehearsal (that really pisses me of!!).
Disclaimer: A lot of the above may be very egg sucky, but it's way I roll :-)