Finland is exceptionally card-friendly. Contactless cards and phone wallets work in taxis, cafes, markets and many places where a visitor might expect cash.
The rounding happens once
Read the cashless sign
Ei käteistä means no cash. A business can choose to operate without cash, so a stack of euros is not a universal backup. The calmer setup is one primary card, a different backup card and a small cash reserve.
Choose EUR at the ATM
If a machine offers to charge your home currency, it is offering dynamic currency conversion. Choosing EUR normally leaves conversion to your own card network and issuer. Read the operator fee before approving the withdrawal.
The shelf price is the real price
Finnish consumer prices already include VAT. You do not need to add sales tax mentally at the register. Cash rounding may move the final cents, but it does not create a new fee.
Cash still earns one small pocket
A rural stop, summer kiosk, market stall or temporary card outage can still make a EUR 5, 10 or 20 note useful. Keep the reserve modest and ask before ordering when payment acceptance is unclear.