Paying with a plastic card in Belarus is a fine thing, no doubt, but there have been a number of cases when the ATM displayed ERROR, or required a PIN code that had been forgotten long before or the card reader in this or that establishment malfunctioned.
My customers seem to be divided into two groups - one sticks to cash payments only, the other - to card payments and sometimes to my great surprise they can tour our province for 3-4 days without having to use cash at all. So you can rely on your plastic card for payments on check-in in most hotels and for paying your restaurant or café bills (after having checked that they accept cards).
A reasonable amount of cash, small denominations – EUR (or USD) 10, 20 or 50 – might be handy to get around in Belarus. Please, bring in bills in good condition - undamaged and clean - because banks in Belarus charge commission to accept (if at all) dirty or worn-out bank notes.
Some of my American guests noted that cash withdrawal from ATMs cost them $5 for each transaction. Check your bank commission before you go!
Having a reader doesn't always mean you can pay with card in Belarus.