Checkpoints aren't that common. So Cal is a big area, and you can go literally years without running into them. I myself have never run into a roadside smog (which are still voluntary, I believe) and I have lived in So Cal my whole life.
And yeah, be polite and respectful and you have almost nothing to worry about. All in all, the biggest thing you can do to deter cops is to have a plain, newer car that blends in, that has everything in order. Tags up to date, no tail lights out, etc. Dark tint on windows = pulled over. Lowered ricemobile with fart can pipe = pulled over. Crappy car with mexican, black, or trashy white guy = pulled over.