I'd put them as guardians, personally. It doesn't take much energy to bring a 2* uncommon to 3* to use as a food.

In this particular situation, I have a bunch of rares I upgraded to 4* 40, but i don't use them for anything.  It would save a lot of time and resourced to upgrade them to 5* to use as food for a 5* 50 as opposed to stargin from a 2* or new 3*.  If they were still at 3* 1 i would just leave them as guardians.  Does that change your opinon? I changed the oprignal post to clarify.

Depends honestly on how much you use rares in the game.

I don't even have or use rare faction guardians at all myself. Just Epics and the couple of dupe legendaries I own. I don't need to be wasting the vault slots for my two Renegades and two Coldhearts and Bellower.

I wouldn't mind seeing which rare champs you have (both using currently and looking to potentially use as food)

My inclination is to feed these rares but I wouldn't mind seeing where you are. 

Thinking of getting rid of 4* 40 Runic Warder Galek Conquror (I have several) verteran

I have at 5* 50or higher: Bellower, War Maiden, Athel, Kael, Kurzad, Dilgol, Apothecary, Soulbound Boyer, Paragon, Pain Keeper, Pantera, Greybeard, Ordinator, Sentinel, Grappler, Scrapper, Reliquary, Haruspex, Seduvcer, Heiress, Theurgist, Metalshaper plus a bunch of Epics and 21 Legendary