Knowing that some brakes squeal until they are broken in, I tell my clients to come back if it does not go away in short order.

Just built myself a bike with dual pivot XT brakes and Kool Stops and it only took a few miles in the summer sun to get the front to stop squealing like a cat in heat but then it was gone.

It is rare for well set up brakes with quality pads to squeal continuously unless they are Avid Shorty 4 (cantis) which are notorious for being impossible to correct... it is fun to pass these on to new mechanics and tell them to get them to stop squealing.

Once they are curled up in a ball, drooling, and rocking themselves back and forth we tell them it is the Kobayashi Maru of bike repairs.