The auto-tuning is often installed to enable the FMS to determine it's position, it's not necessarily for your use. So even when flying point-to-point using GPS, your "Plan B" is running in the background. Your plan should give you the ability to immediately know where you are, if you don't already know, following FMS failure. A radial and DME plus a chart is all you'll need to determine your position. And your position determines things like MSA, airspace limitations, local rules & procedures etc.
PM