The ShiftstonedEternal Power Calculator is a deck-building tool that visualizes your odds of drawing enough Influence and Power based specifically on the cards found in your deck.
I play zero shadow sigils in my list. 16 shadow from duals is usually enough. occasionally you won't be able to desecrate on 2, but going up to 18 sources won't change the math on that much
I actually thought about that, too, but having two Shadow Sigils can still be useful when the opponent uses Ice Bolt or Royal Decree themselves. When I am at 5 Primal influence and get ice-bolted, I want to be able to fetch a Shadow Sigil for ChaCu. But I think this a little bit a question of personal preference.