Just crossed this and I'll clear up a few things........DEF will crystalize if left out or the tank goes dry. The crystals will dissolve once more (liquid) DEF is added. The DEF will freeze. It's unavoidable in cold weather. The tanks/ pumps and lines are designed for this. Once the engine is shut down you will hear a pump run, that's the DEF being pumped back to tank so it doesn't freeze in the lines or pump. The tanks are designed so they can't be filled to the top to allow the freezing DEF somewhere to expand. DEF has a shelf life, which on the upper end is around 2 years in perfect conditions, approx. 60-65F. Anything higher will reduce shelf life down to approx. 2 months at 104F ambient.
In my 6.7 Ford I wait until it tells me it wants DEF, and then add 2.5 gals. My work truck (12 Pete with PX8) I add 5 gals once the gauge hits 1/4 (10 gal tank). No need to keep the tanks full.
2013 F350 dually
2015 Palomino Puma 351THSS Premier Edition
2015 Nights camped 15
2016 Nights camped 5