No, -san is just something you say to someone you don't know well, or just met. It's like Mister, in English. And if I didn't use it, honestly? I'd feel a little strange.

[Rei eyed him up and down as she hopped up the wooden stairs and slipped out of her shoes before entering the little tatami room. He didn't look terribly comfortable, and that was a bit odd. Maybe he just wasn't used to other cultures? That's what she assumes, anyway. Not knowing he's a spaceman and all.]

When you come in, please take off your shoes. It's something of a rule about Japanese buildings. And please, make yourself comfortable over by the fire. That's where we'll do the fortune telling.