A theory is that residue of cleaning agents can cause a squeal, in addition to oils from the road. So the first try IMO should always be cleaning the rim and pads with something that won't leave any. I'd probably scrape the pads though that may be excessive. After that, vibrations can be the culprit which can be due to type of pads, mounting, or I'd say even if the rim is wavy from wear or being out of true. In the latter case I think you'd feel the brakes "shudder" as well as hearing the squealing.