You could, but personally, I like keeping one copy of all of them in my vault, plus filling up my guardian hall with dupes to boost the others.
The way I look at it is - a rare champ is worth about 68 energy (for me, 4 nightmare campaign + 1 normal campaign brings three 2* champs to level 20), which is about three hours worth of energy. Is it really worth eating a rare champ that you may want later?
Whats the guardian hall? Dont think I've seen that.
And yeah, I'll definitely keep one copy of my rares, my question was more regarding dupes. Especially ones with fusion symbols. Specifically Sorceress and Skullsworn for me now. Sorceress I fave fully ascended to 40 for the fusion requirement + 2 copies. What should I do with them?