I use quite a bit of them for food. Two uncommons are worth saving in my opinion, Armiger and Shieldguard. Only a few rares. 

Hellhades did a recent and great video on the rares to keep right here. He also has some videos on the Epics to keep.

I save 1 of all epic and legend to own them all

I use rare for 5 star chickens (some very few exceptions)

Below rare are just burned as XP for other champions

Not enough room to keep 1 of every rare, so most get eaten.  HOWEVER, I recommend keeping ones that could potentially go on a FW team, so make sure to do your research.  Also keep any fusion rares (they have the little round symbol on them).

I do keep at least 1 of every epic (rest are books) and any lego I get (in case a dupe system ever gets put in).

i feed 5 epics which i class as mid to end game champs at 50 with gear to make a 6 star champ over the weekend, i did this for progression at some point ill get those 5 back and with 20 sacreds stocked up for the 2x sacreds this weekend who knows i may get some luck and pull a couple of lego replacements thatll help me wipe those areas im stuck at currently.

im finding some areas im 1 champ from wiping and others 5 champs from wiping but its also alot to do with finding the right gear to upgrade my currect roster