Bays are normally always out of level, the shape and weight of the windows mean they drop in the middle.
When we measure we use a laser and take all of this into consideration. However, we are still fitting a straight level shutter frame into a recess that is generally not straight or level.
This is why we use a shadow gap, to distract the eye from seeing the gap, so it focuses on the neat shutter frame.
If your window is out of level, you’re going to see it if you look hard enough. If you use trim to fill the gap, it will be cut and tapered to fit, if you use filler, it would fill the gap, but you would still see the trim or filler getter larger at one end.
The shadow gap does the same job, but just uses a natural shadow, the advantage is that it doesn’t crack or discolour.
So generally, there is no magic option, it’s just being aware that the bay is out of level and that you would have varying shadow gaps.
If you’re having building work, so customers have re-levelled their bay windows ready for their new shutters. If you’re going to do this, it would need to be done before final sizes are taken.