Generally populated areas such as Southern California have coordination groups which help maintain repeater frequency pairs, assignments, etc. This ensures less interference from nearby frequencies, and allows repeaters to maintain their frequency pairs.
The coordinators provide a very positive experience for ham operators and repeater owners.
Here are the coordinators for So Cal
2 Meters – Tasma http://www.tasma.org/
220 Mhz – 220SMA https://www.220sma.org/
29, 51, 420, 902, 1240MHz – SCRRBA http://www.scrrba.org/