I read somewhere that creosote buildup is basically like having a layer of tar inside your chimney, and once it catches, it burns hot enough to warp metal flues. Blake's right, it doesn't take much to get that stuff going... I remember a news story about a family who lost their house because they skipped a cleaning for just one season. That shortcut might save you an hour now, but it's playing with fire in the worst way.
Yeah I read an article last week that said the same thing, @blakehenderson. It was saying creosote buildup is no joke and can flash over real quick if it gets hot enough. Definitely makes you want to get the chimney cleaned more often.