You should always select a frequency above the -3dB point of the speakers. Thus, if the speakers are rated 60-20,000Hz +/-3dB, you should select a frequency above 60Hz, because it is already on the downward slope of its frequency response curve at that point, and is not as capable at that frequency as it is above it. I have bookshelf speakers rated 50-40,000Hz +/-3dB, and I use 80Hz for the crossover point.
Exactly how much above the -3dB point will depend on the particular speakers, and also on how hard you plan on driving them. Basically, the louder you want it, the higher the frequency should be, to keep the woofer from bottoming out. This is assuming that the subwoofer can handle things, of course.
If one frequency were always the best choice, there would be no point in having that aspect of the performance adjustable.