When I picked my call, it was both an oblique reference to location, and to the construction materials (PVC) used to build the antenna.
It also makes sense to choose an ident that is easy(er) to read as a weak signal. This involves care in the choice of letters to get a sensible ratio and juxtaposition of dots and dashes. For example HI would be a pretty poor choice (.... ..) , whereas XO (-..- ---) might work quite well, if one is using simple keyed CW. For FSK, things are a bit easier, because the carrier is always on.
Choosing a frequency has a number of aspects. The first is doing one's best to stay away from the centre of the band at 13,560 kHz. There is a lot of commercial interference here, so it is much harder for a mW power beacon to be heard in the QRM. The second factor is the adjustment range available with the setup, especially if trying to "pull" a crystal oscillator. This could determine whether the transmitter is above or below the band centre. Lastly, of course, it is polite not to land on top of another beacon, but one can afford to think really narrow, and 100 Hz, perhaps less, is a decent margin. Listening is often via spectrum analyser, so as long as traces don't overlap, they can be picked out. There are a couple of beacons that are well known for drifting up and down in frequency as the local temperature changes. There are no hard and fast rules, aside from working within the band limits, and not exceeding the maximum allowable field strength, but there are some good tactics to enhance one's chance of getting some reports.
Cheers
Ed