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