Interesting to hear about different practices!
Since I posted my question, things have developed here - Castor has grown (six months now) and can jump up on beds and couch quite easily.
My desk is next to our bed, and when I work from home and sit by the computer - but only then - Castor is allowed on the bed. So far, it has worked out very well; he actually seems to understand that he can only ask to go up onto the bed if I'm sitting there.
He's not allowed on the couch, so when we're not at home, we arrange the cushions so that he cannot get onto the couch (and we close the bedroom doors).