Ah ha - I am ahead of the game (not really). I have just ordered new white venetian blinds to replace the blue vertical blinds in the kitchen. They were here when we moved in 20 years ago, are probably 30 years old. The did have additional dress curtains, and pelmets when we moved in, which were ripped out on day one.
Just about to change curtains too. We brought some with us, which are now 30 years old, and some were put in when we moved, so 20 years old. Honestly, there isn't much wrong with them, just a bit of sun bleaching. They are simple plain curtains, I am just bored of them.
So, my bay window, with a radiator under has sill length. When we moved in it had sill length curtains in the bay, with additionally, dress curtains over the big opening, along with swags, and pelmets. I tell you, the first thing we did was rip those out. Made the room look twice and size and twice as bright. So, for me, dress curtains are excess clutter and cut out light for no purpose. They would be the first thing to go.
So, I am afraid I am going with sill length in the bay again. Sorry.