Ever. Ever ever.
Mortal psychology is inherently trying to game the system. It forgets favours done and remember slights inflicted, in the hopes that it won't have to pay back the favour. Because the system is inherently gamed, you can't use it.
Always assume, if someone is asking you for a favour, they intend to forget the favour.
As always, theft destroys the producer of the stolen property.
In other words unless the favour costs 0, such as something you were going to do anyway, you shouldn't do a favour. Charge for it or tell them to fuck off. Asking for a favour is inherently a scam. Doing a favour is inherently simping.
I guess you're allowed to do stuff for free, but don't call it a favour. In fact assume it's going to have additional hidden costs; only do it if you're willing to pay for the privilege of having the opportunity to give stuff away.
It's fine to ask for favours though. You should milk everyone you meet for as many favours as possible. Apparently they like it; it makes them feel superior. It's fine to keep going until they start refusing. Don't do anything stupid like remembering the favours they did, because they wouldn't. Don't cooperate with defectors. They're Democratic Man; they probably won't even remember having done the favours. They instead do a reverse thing; I did a favour for them, therefore I must like them. They only remember feeling good about you. Only don't ask for favours if in some way the request itself costs more than the gained resources are worth.
I get offered free stuff fairly regularly without even asking. I consider it tribute. Naturally, I am owed sacrifices by virtue of just being that awesome. Be like me.
Sadly I remember all the favours. It doesn't work. This is actively counterproductive. Don't be like me.
If someone is dumb enough to offer favours, then the only thing worse than you "unjustly" absorbing their favours is someone even more deviant and dishonourable absorbing the favours, which is what will happen if you don't part the unhappy money from its abusive fool.