Hands up those who fly with GNS430/530 or glass cockpit but also carry a handheld GPS. Not just for backup but because it's easier to use or perhaps you are more familiar with it.
DO.
I have a Garmin 496 yoke mounted, mainly to get the TAWS warnings through the intercom, but also to act as a last resort battery powered backup for a total loss of electrics. I still have an old Skymap 2 in a bag but the 496 is right there all the time.
Due to its poor interface, the 496 is much harder to use (load a route into) than the panel mount stuff I have (KLN94/KMD550) but is not bad for a DCT - DCT - DCT route which is all one would do in an emergency, especially under IFR.