Yes, I don't even bother with FG for rares 

Personally, I like keeping one of each rare just in case they end up being useful. But I'm a hoarder.

Realistically, a rare champ is worth roughly 224 energy. For me, that is nothing, but for newer players that's a different story.

224? Curious on that calculation... seems high

Also don't  forget the DT rooms  and the ones you can use as books in cvc, everything else is food.  

Fair enough - forgot to mention that math was using solo farming dungeons, so the XP per energy is lower. If you run Brutal 12-3 instead, it's far lower - only 75 energy.

I only have a very few noteworthy rares. Some out of history like Apo, Kael and Bellower, others who actually are played in some content (Coldheart, Souldbound, ReliTender and Painkeeper),,,but for 95%, they are just fodder

I'm a mid game player that had a similar problem recently, and I'll tell you what I would look for: 

• Obviously Rhazin fusion champs. 
• Any reviver that could be used for Faction Wars - some rares are actually quite great at level 40 or 50 in FW as a sole reviver (Avir the Archmage [Dwarves], Geargrinder [Orgyn], Haruspex [Lizardmen] and any similar champs (make sure to read skills. 
• There's also other "good" Faction War champs that can be helpful to you, but should not ever be valued past FW champs. Mother Superior, Hexweaver, etc.
• Duplicate champions for Faction Guardians that are only specific to top quality rares. So, getting your Dark Elves Guardians to be full for your Coldheart, or your High Elves to be full for your Apothecary. Whatever top quality rares you might use, I would use Faction Guardians only to boost them. No other reason to keep other sets of Faction Guardians. 
• Particularly solid rares like: Frozen Banshee, Doomscreech, Soulbond Bowyer, Apothecary, Bellower, Reliquary Tender, Coffin Smasher, etc.
• Late game Clan Boss teams: Painkeeper, Hellborn Sprite, Corpulent Cadaver

I've been reading the character reviews to be sure I wasn't missing something - but it looks as though one person has given possibly every rare a 5 star review for Iron Twins! Possibly also Hydra and Sand Devil. I've therefore held quite a few back because when in doubt, don't. :)

FWIW - here are the rare champs I have at 60, and the reason I leveled them:

• Coldheart - Spider, FK, DTHSR
• Hexweaver - DTHSR
• Soulbound Bowyer - DTHSR
• Metalshaper - Scarab King
• Bellower - DTHSR
• Frozen Banshee - CB, DTHSR
• Elhain - Starter, DTHSR
• Bloodmask - DTHSR
• Reliquary Tender - Nether Spider, DTHSR
• Master Butcher - Bommal
• Apothecary - Early Dungeon support
• Geargrinder - DTHSR
• Doomscreech - DTHSR
• Executioner - DTHSR
• Gnarlhorn - DTHSR
• Pain Keeper - DTHSR
• Coffin Smasher - Early CB team, DTHSR
• Diabolist - Early Dungeon support, DTHSR
• Warmaiden - DTHSR
• Kael - DTHSR

You should worry somewhat about void rares. I got a renegade really early and fed her. I recognized some potential but was like "meh, It'll be months before I need something like this, I'll just get another copy". Well, it took almost a year to get that 2nd copy.

The colored rares are much easier to get again. Ironically, rares become useless in the midgame and then climb up in value again for the lategame. So I'd just feed all that don't have obvious value right now. Then, just forget about the topic until you hit the mid-late game (~6-9 months in), where the value of chickens starts dropping a lot.