I am posting this, just to share my experience.
The forex charges are applied by your bank, and in my case it is 5.99 EUR each time I withdrw money from ATM, and around 1.50 EUR each time I pay using my card. Thus, withdrawing a large sum, depending on your need, might be beneficial for you.
I withdrew money from Česká spořitelna ATM at Praha Main Train Station.
One point to be noted, as also pointed by @trollster, is that I got 2 options while withdrawing the money:
- Withdraw without conversion (to your base currency)
- Withdraw with conversion (to your base currency)
I saw that for the later, the exchange rate is pretty bad, so I went with the former one, assuming it won't be worse than this. With this choice, I got approximately 27.02 CZK/EUR exchange rate from the withdrawal (which might be unrelated with the choice of ATM you have), which is also the official rate at this moment.
Apart from this, I also exchanged some EURs, I was carrying, from a place near Národní třída, named something like (Ex?)Change 0% Commission. I got an exchange rate of 26.20 CZK/EUR there without any additional charges, and was able to exchange small amount of euros such 20-25.
Summing up, it is best to know about the forex charges of your bank beforehand and then withdraw money from ATM, when withdrawing large sum.