it does not work this way. If it is DR and other independent navigation - it must be done every time, with full precision and procedures, not only practiced when you have time and resources, like a funny toy. Or at the moment when everything collapsed you are in the same nowhere. So it is obvious unreal, it rises the cost (efforts, time spent, risks due to distraction from control, requirements for track planning etc) so high that the whole process becomes impossible. Its like to always carry full month survival kit with you, 50 kg backpack even if you are walking to grocery store.Which is a good reason to have a practice every few flights.
So all these emergency procedures must work without prehistory, ie they are obligatory absolute means dependable - GPS, radio beacons etc. You fear that your panel GPS fails? take 5 more GPSes with you - tablet, mobile phone, wrist watch, radio and fitness bracelet, everything now has built-in GPS. Even if it does not give precise information, real map and course directly to home door it will be enough to be not lost. At least you find the way to somewhere, not obligatory to where you wanted, but to safety.
And in the case of global GPS fault (sudden nuclear war with space explosions, killing all satellites in minutes without any preliminary signs) navigation will be not the most significant problem.