There are very few common and uncommon champions that are worth keeping, and the few that are can easily be replaced even if you accidentally get rid of one. Even a lot of the rare ones are expendable, but you'll want to be a bit more careful with those, especially in the early game, since they can fill certain roles you may lack better options for. You'll be swimming in green shards soon enough though, so use common/uncommon ones for food as much as you want.

I do not think 1* and 2* champs are good for general purpose, but there are some that are special purpose like clan boss and spider.  If you check the ratings you will be able to pick those out, like Sister Militant, OutlawMonk, Deathhound, Armiger, Shieldguard, Saurus.  It you play often in the campaigns (or summon your green shards) that offer those you will get some duplicates which you can use to boost that champs's skills.

3* are similar story, if you are not yet familiar with many of them keep checking their reviews and skills, and track and collect champs that represent as many different skills as possible. So duplicate skill champs you can eliminate as food if you like.

There  are  very  few  uncommon  champions  that  your  account  should  ever  build  out,  and  none  of  the  commons  are  worth  it  either.  Plus,  commons  can't  wear  amulets  or  banners.  I  would  say  there  Is  one  uncommon  champion  an  account  should  ALWAYS  build.  Armiger.  Otherwise,  some  uncommons  may  be  useful,  but  may  depend  on  account  to  account,  like  Shieldguard,  Elfguard,  and  Zephyr  Sniper.  I  agree  that  commons  and  uncommons  are  easily  replaced  and  should  be  fooded,

Hello @Aquila 

It depends on your Account level and where are you in the game, usually you can sacrifice alll the 1 and 2* Champs, (expect Armiger he worth leveling) and some Rare are worth Leveling and could be very helpful for your account progression. 

Alpha H