After extensive testing as far as just raising their affection level goes I don't think it matters whether you fill the bar or not. The bar just changes their mood so you can get bed scenes.
The only thing that seems to actually affect affection is whenever you see the orange music notes pop out of their heads. This includes praising them during battle, buying them food, etc. I maxed one of them in the middle of a chat (the trophy popped) after the bar was already full and some orange notes came out.
Yeah, but chatting in bed before the bed CG seems to boost the affection meter faster.