That applies to anything though. Rely on a routesheet and go off-route, and you're fucked. Rely on a paper map and have it destroyed by Weather and you're fucked. Rely on someone who only claims to know where they're going and you're fucked (but blissfully unaware for longer).
For values of 'fucked' that may or may not be actually that important. 90% of the time it's only a bike ride, and a lot of that you might be able to wing it successfully using roadsigns and a vague idea of where you're going. If it actually really matters, and you're not an idiot, then you'll be carrying redundant navigation solutions. For audax rides, I usually have a copy of the routesheet with me as well as my Garmin. For touring, I tend to have some sort of map (and usually a computing device that can be pressed into navigation duty at a pinch) as well as the Garmin. For local rides, I have a reasonable idea of where I am, and the Garmin's just data logging.