Keep all rares for now, use uncommons as chickens.  You can always use a lvl 40 rare then turn into a 5 star chicken 

The short answer is "no". There are a very small number of rares that retain value into late game:

• Coldheart (Spider, Fire Knight (FK), Scarab King)
• Renegade (Clan Boss (CB))
• Pain Keeper (CB)
• Frozen Banshee (Dragon, CB)

There are a few others that are situationally useful, depending on your account:

• Apothecary (if you need a SPD up champ)
• Bellower (if you need a campaign farmer)
• Fellhound (if you need a campaign farmer)

After that, the remaining value from rare champs comes from two areas - Faction Wars (FW) and Doom Tower Secret Rooms, specifically, on hard (DTHSR):

• Metalshaper (Scarab King)
• Reliquary Tender (Nether Spider)
• Corpulent Cadaver (extremely niche CB team)
• Templar (Bommal)
• Soulbound Bowyer (DTHSR5 Rotation 1, (DTHSR4 Rotation 2, DTHSR9 Rotation 2)
• Warmaiden (DTHSR5 Rotation 1)
• Your starter rare (DTHSR5 Rotation 1, DTHSR9 Rotation 2)
• Hexweaver (DTHSR4 Rotation 2)
• Gnarlhorn (DTHSR4 Rotation 2, DTHSR7 Rotation 3)
• Diabolist (DTHSR12 Rotation 2)
• Executioner (DTHSR12 Rotation 2)
• Doomscreech (DTHSR7 Rotation 3)
• Geargrinder (DTHSR7 Rotation 3)
• Doomscreech (DTHSR7 Rotation 3)
• Bloodmask (DTHSR7 Rotation 3)
• For FW rares, pay special attention to any that have revives or CC abilities. Those are the ones that could have value. A good example is Gnarlhorn.

Posting this on my alt acct since my main one is banned and I don't want to lose my clipboard :p

The short answer is "no". I still keep a copy of all my rares because you never know when you'll need one, and I have all my rare faction guardians filled, but beyond that, there are a very small number of rares that retain value into late game:

• Coldheart (Spider, Fire Knight (FK), Scarab King)
• Renegade (Clan Boss (CB))
• Pain Keeper (CB)
• Frozen Banshee (Dragon, CB)

There are a few others that are situationally useful, depending on your account:

• Apothecary (if you need a SPD up champ)
• Bellower (if you need a campaign farmer)
• Fellhound (if you need a campaign farmer)

After that, the remaining value from rare champs comes from two areas - Faction Wars (FW) and Doom Tower Secret Rooms, specifically, on hard (DTHSR):

• Metalshaper (Scarab King)
• Reliquary Tender (Nether Spider)
• Corpulent Cadaver (extremely niche CB team)
• Templar (Bommal)
• Soulbound Bowyer (DTHSR5 Rotation 1, (DTHSR4 Rotation 2, DTHSR9 Rotation 2)
• Warmaiden (DTHSR5 Rotation 1)
• Your starter rare (DTHSR5 Rotation 1, DTHSR9 Rotation 2)
• Hexweaver (DTHSR4 Rotation 2)
• Gnarlhorn (DTHSR4 Rotation 2, DTHSR7 Rotation 3)
• Diabolist (DTHSR12 Rotation 2)
• Executioner (DTHSR12 Rotation 2)
• Doomscreech (DTHSR7 Rotation 3)
• Geargrinder (DTHSR7 Rotation 3)
• Doomscreech (DTHSR7 Rotation 3)
• Bloodmask (DTHSR7 Rotation 3)
• For FW rares, pay special attention to any that have revives or CC abilities. Those are the ones that could have value. A good example is Gnarlhorn.

The  later  part  here  I  don't  quite  understand.  All  of  the  champions  (like  Hexweaver,  Sorceress,  etc)  for  Doom  Tower  Secret  Rooms...are  those  required  champions  to  play  that  room  on  hard  or  what?  

So  for  instance,  I  ranked  up  a  Lifetaker  to  Rank  5  to  use  as  fodder,  should  I  actually  be  keeping  her?  I  was  under  the  impression  that  *most*  rares  didn't  reallt  matter  outside  of  some  of  the  top  tier  ones.

The Doom Tower secret rooms all have very specific requirements. The ones above that I mention are usually things like "Use only Rare HP champs". That's why they have value.

You're mostly correct. The reason why Trips is suggesting you save them is because statements like mine earlier are based on you getting better epics or legos in place of them.

But you could end up with just terrible luck, and not get any epic champs with ATK down for example, at which point you may need to use a rare champ on your CB team just until you eventually do.

Eventually you'll use those rare champs as food, likely. But in the interim, the difference between a rare champ and an uncommon champ is only about a hundred energy that it costs you to level that uncommon champ to 20 and turn it into a 3*. That energy cost does not warrant losing the rare that you may end up needing to progress.