Painting help

[Waitingtomove]

We have recently moved in to a new house which is about to be painted. Im planning on having white ceilings with shadow white on the walls. There are some double height spaces where the wall curves in to the ceiling so there is no natural place for the wall colour to stop and the ceiling to begin.
The decorator suggested masking a line on these walls but I'm really not keen on this. So do I paint these walls colour or white... or something else?

I think the answer will depend on the overall set up of the space. The proportions between walls and ceiling in the space, whether the ceilings connect onto other ceilings, whether it would look ok or weird to have the odd white wall in those places. Sorry that's not much help.

I think masking a line below the start of the curve might be ok with a different colour, but with shadow white it's a bit odd. It's not quite different enough to make sense.

Is there a feature of some sort on the 'curved' wall at all? We have ceiling curving onto walls that have windows, so we've put a line level with the top of the widnow recess. Originally, all rooms in the house had picture rails (removed long before we moved in), so that was a natural divider.