I've always rad to lightly prune them after the blooms are spent.
In California, pruning lavender depends on the bloom time of the plants. There are good tips here:
generally lavenders are pruned once a year. Since California is warm, you could prune in the fall. Once the blooms have dried and gone grey, prune down a good third. You can even go a bit lower just don't go too far into the old wood. Givethem a nice rounded bowl cut for great shape next season.