The BBC article SpaceX aims to launch internet from space discusses the SpaceX proposal for a dense array of 4,425 LEO satellites distributed among 83 orbital planes in five inclination groups as described in the linked document SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S.
The altitudes of the five groups are all between 1110 and 1325 kilometers. Within each alt/inc group, collisions can be avoided by adjusting the phasing between planes. While there is nodal precession, it will be essentially identical for the entire group as nicely pointed out in this excellent, well thought-out answer. However, each inclination group will precess differently, so in order to avoid a nightmare of stationkeeping, the inclination groups are placed at slightly different altitudes.
I don't think the particular values of these five altitudes were selected randomly, but I don't see any compelling orbital mechanics arguments. So I'm thinking they were chosen to avoid stuff that's already up there in established orbits.
So I'm wondering for constellations of circular LEO satellites, are there allocations of available "slots" in altitudes? Assuming these were chosen for that reason, were they assigned, or allocated, or just dreamed up by SpaceX on-the-fly?
above: screen captures from SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S.