i can't get into the churning thing. i use my credit union's card as most people would use a debit card-i just don't want to have to keep track of/write down the debits in my check ledger/for tax records when the credit card keeps better track. i do earn points on it and get some discounts but we really only got it when we were going to canada and learned they did foreign transactions/atm withdrawls free of charge

the other 3 cards?
amazon cuz it's basicly giving me 5% off every purchase AND amazon is one of the most common phising purchases fraudsters put on debit/credit cards so if one shows on my other accounts it's an automatic tip-off. costco-cuz...it's costco. amex b/c they will fight tooth and nail for their customers on disputes. i'm still surprised i get rewards on a no fee card from them but i set up my streaming services and other autopays with them so i end up with a bit over the course of each year.
on having a backup credit card while traveling-YES, YES, YES!!! we had a nightmare some years ago that was entirely the fault of
disney cruise lines that caused a mess with the sole credit card we had on us. the credit card company tried to facilitate getting it cleared up with disney but apparantly all their charging services are done off board and they have to communicate via email....took weeks to clear up. i will never travel without a backup card again.