I have a divergent opinion: cross-chaining is not a big deal. Nothing is gonna happen if you're big/big or small/small. Don't worry about, and use all the gear you've got to do the most work you can. On many bikes, the big/big will be noisy; fine, use it briefly and don't hang out in that gear.

The trick with gear selection is to choose the chainring that allows you the widest working range for the conditions, so if you find yourself cross-chaining often, you're probably missing the opportunity to change chainring earlier. But, it depends on terrain, speed, duration of need for the cross-gear, strength, etc.

As for where you should be chainring-wise, that depends on your strength and fitness. If you can't push the big ring without getting whipped, use the inner ring. It's kinda simple in some senses.

but remember, if your drivetrain is properly setup and in tune, cross chaining isn't going to do anything bad, and those gears are there to be used. If you don't use 'em, lose 'em! Put the machine to your service, and make it do what you want to do.