honestly, i wouldn't bother. once you hit a certain point in the game your supply of rare tomes will well outstrip your demand for them and its not worth spending a bunch or resources just to give yourself an opportunity to use a few books. even if you don't have any tomes to use on them, you'll get far more benefit out of bringing someone like rector or sandlashed to 60 than you would from a fully booked rare.

if you ever hit a point where you are fully focused on faction wars, you may want to revisit this question (though by the time that happens you also might have enough epic or legendary champs to make it unnecessary) but until that happens i'd generally stick to focusing on better options. of the ones you specifically listed, the only ones i'd consider properly useful are soulbond, paragorn (who can do his job just as easily at level 30), and maybe felhound.

Thank you for the insight. I am planning on ranking up Fell Hound just to get a super fast campaign farmer. Other than that I'm probably going to prioritize the epics you mentioned. Thanks again!

By far the biggest area of use for them is the dt hard secret rooms that require rare Champs. Keep an eye on those and who you plan on using for them. If you're too early for that, then just save them for now. You'll need a lot of them. 

-  Seducer

-  Metal  shaper

-  Fell  Hound

-  Gnarl  Horn

-  Hound  Spawn

-  Diabolist 

-  Marked

-  Conqueror

-  Bulwark

-  Runic  

-  Executioner

-  Abyssal

-  Soulbowyer

-  Skull sworn

-  Bog  walker

-  Bersker

-  Paragon

-  Harapex

-  Witness

-  Wretch

I  have  seen  people upgrade  all  the  above  Rares.

It  doesn't  mean  you  have  to,  but  its  just  fruit  for  thought.

I would also look to be saving those food resources for the Epics and Legendaries I pulled. You already have a good stable of the best rare champs ranked, including my personal favorite Coffin Smasher :D, and as you get to Hard Doom Tower Secret Rooms that require Rare only champs of specific types, you can plan out those upgrades to not upgrade someone you will never use.

Epic champs like Dhukk, Burangiri, Sandlashed are really good, and can see a ton of use for you in both FW and Doom Tower, and Sandlashed is a super good Arena champ besides.

I would look at Visix, Cybele after that. Then potentially move onto champs like Hope and Deliana and Aina for FW.

I can certainly see getting Fellhound ranked up. A 6-7 second farmer is nice. :)