AirBnB has some good high level thoughts on the topic, for example - https://www.airbnb.com/support/article/695?topic=229

Couch Surfing (as distinct from renting someone else's space entirely for yourself) probably warrants some additional investment in the relationship and ensuring all expectations are clear.

I've also heard of many guests who budget for some kind of thank you gift - my Australian friends usually take a pack of clip-on-Koalas to hand around, and a bottle of wine or flowers are usually safe bets. You'll be saving more than the E10-20 they cost you, so it's a good investment in being awesome.