Well, this is something you don't ask for an opinion on. It is really determined (demanded) by the speakers you are using.
If the mains speakers have a -3dB down response to 40Hz (for example), you would set the crossover at an octave above that. So they would have to be extremely capable speakers to choose a crossover below 80Hz. That's the preferred crossover. You'll obviously have less equalization control if anything less is chosen.
Subwoofer crossovers are forced at the upper end, so generally set at a fourth order LP and so -24dB per octave would be suitable. For the mains which enjoy a natural falling off at the lower end, then a second order -12dB per octave would be correct.
The cutoff in REW is always set to match the crossover frequency.
brucek