More is lost to spreads than to the exchange rate. Where the cost actually sits.
The rate in the news is the interbank rate; what individuals actually buy and sell at is worse. That gap is the spread, and it varies a lot by currency and by counter. It is why changing the same amount on the same day gives different results depending on where you go. Compare the final amount of foreign currency you receive, not the quoted rate.
Online exchange through your main bank usually offers the best rate, and airport counters the worst. Change only what you need for transport at the airport, and arrange the rest in advance or locally.
Changing everything to cash creates loss risk and the cost of converting leftovers back. In countries where cards are widely accepted, carry cash for small payments and use a card for the rest. Cards carry foreign transaction fees, so knowing which of yours charges least makes a real difference.
Abroad, a terminal may ask whether you want to pay in your home currency. The rate is set by the merchant's side, usually to your disadvantage. Paying in the local currency is almost always better.
Coins are generally not exchanged back, so spend them before leaving. Notes lose to the spread a second time when you sell them, so you pay twice. If another trip is likely, simply keeping them is reasonable; airport donation boxes are another option.
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