Today's breakfast was 1,35€. I tried to pay 2,15 (1+1+0.10+0.05) so I get back 0.70€ (0.50+0.20), so I have less coins in my pocket, and the cafeteria has more coins to give back to future customers.

Well, it seems I am a weird person for doing that. My friends got surprised ans said Why don't you give 2€ (1+1) and that's all? The bartender also looked at me a bit confused, and took only the 2€ (and gave me 0.50+0.10+0.05).

When I told my friends that I do similar things in every shop, they said that cashiers hate me for sure O.o

Am I supposed to use coins only when they make the exact total? That's boring IMHO (and I would end with too many coins in my pocket).