MOST of us know this and do the right thing.
One of the responsibilities we have as hams is to help other hams out.
Some of us collect paper, some don't.
Some are going for awards, some are not.
The hobby is different things to different people, still ALL of us have a responsibility.
It is the responsibility of the person that needs a QSL card to send the an SASE or for DX the proper postage and an SAE. These are the rules.
It is also the responsibility of EVERY ham to respond to every ham in a timely manner that plays by the rules and provides postage. A card is not needed, but a written proof of QSO is. A note is sufficient.
Another thing.
If a guy is CQing a specific whatever it is bad form to answer him unless you ARE the specific whatever...UNLESS you need him for an award.
If you are the CQing station and someone specifies he needs you for an award it is good juju to reply even if it is not what/where you are looking for.
If someone is CQing for a specific whatever and you are not what he is looking for then leave him alone.