By law you have to agree to change rounding. If I'm asked for €50.50 instead of €50.48 I don't know that my bill is €50.48 until I get the receipt. I should be asked for €50.48 and given €49.50 change if I tender a €100 note but I should have the option to tender €50.48 in full settlement of the debt.
I've noticed that An Post don't subscribe to change rounding at all, they still give out 1c and 2c coins in the change.