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.
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