This decision also depends on the listening room, the shape of the FR of your sub(s) vs. your main speakers in the lower frequencies, and whether you're using digital room correction ("DRC", like Audyssey, ARC, or similar).
Without any DRC:
It's very likely that your carefully located, single subwoofer will produce notably smoother bass than your main speakers. This is because you can place the sub close to a wall where reflections of low frequency, long wavelength signals (destructive interference) are minimized. If you are using a pair of subs, their output is almost surely much, much smoother in the bass because, while each sub will still have small FR irregularities, those irregularities are likely at different frequencies and will tend to average out to a very smooth summed response (vs bass from main speakers out on the floor). IME, this difference will usually be pretty dramatic.
In this case, cross the subs just high enough to allow the subs to handle response where the room does its worst damage. In an effectively treated room (bass busters, etc), I've found that this will generally be somewhere around 70 to 80 hz, although it's obviously dependent on the characteristics of your specific room. In an untreated room, the worst issues usually persist up to +/- 125ish hz. I'd suggest that you experiment with frequencies between 70 and 100 hertz and see what sounds best.
If you're using DRC, then it's a different story. IME, the software will effectively smooth bass response wherever the subs are placed and wherever they are crossed. In this case, you have a lot of flexibility and the choice will be more dependent on the specific qualities of your subwoofer(s) vs your main speakers. Try it at 50hz and try it at 80 hz and see which you prefer.
Good Luck
Marty
