FWIW, I found this posted a long time ago and saved it for reference. Unfortunately, I lost the attribution so I can't properly credit the author of the post. Suffice to say, it isn't me but the information has been useful over the years.
"A little experimentation does not hurt at all, as different people hear things differently. So experiment with few different crossover frequencies (depending on speakers FR range) and keep the one, you like best.
Having said that ideally and for a seamless integration of mains and sub, they should overlap each other by one full octave. As you know the crossover is not something where main speaker will stop at 80hz and sub will take over from 79.999hz. The mains will still get contents below crossover frequency and sub will also get some contents above crossover frequency. So your main speakers should be flat (+/- 3dB) for a full octave below crossover and sub should be flat for a full octave above crossover frequency. In this example, if crossover is set at 80hz, main speakers should be flat up to 40hz and sub should be flat up to 160hz. This way any spill over content, will still be reproduced with good amount of accuracy by either or both mains/sub.
The receivers which allow you crossover setting for individual channels, you have more flexibility, but for receiver which have just one crossover frequency for all channels - it is a good idea to take the smallest speaker in your setup and set crossover keeping its FR in mind. Not doing so will result in loosing the contents, which are sent to those speakers below the crossover frequency."