Recommended but not required. I've been switching between tunes without going back to stock with no problems whatsoever. The only time it would be required to go back to stock would be if you're updating your tuner's firmware or loading new/updated tunes to your tuner. Not going back to stock when doing either of those is when people tend to run into issues and sometimes brick their ecu.
2012 Kona Blue 3.7L V6:
Borla LT headers, O/R h-pipe, Borla S-types, 3.73 gears,
Steeda adjustable UCA w/ mount, BMR LCAs, Airaid CAI,
LCA Relocation brackets, AE driveshaft, GT 80mm TB,
Black 18x9/10 2010 GT500 Wheels w/ Nitto NT555s,
Eibach Sportline Springs/shocks & Koni STR.T Struts