I do deviate from it, but I'd be hard pressed to give a percentage of how often it happens. City placement suggestion seems to be heavily influenced by resources, which as a general rule isn't a bad thing, but there are forced that the AI can't figure in. For example, wanting to box out a neighbor, going to a resource that I don't have access to but is technically not as rich of an area, or sometimes just because the find the wierdest places to find cities sometimes. I find it especially negligent when on a islands or small continents, the AI doesn't take into consideration the optimal placement to get the most out of the whole island, versus just the one city you are currently placing.