I have a conventional concave cornice and I get plenty of cobwebs in the 10mm step out from the wall plaster (but not so much the step out from the ceiling plaster, or maybe it doesn't show so much there). If you are replacing the ceiling I would go with a shadowline rather than square-set which will show every wobble in your plaster or paint. I don't think it is feasible to install a shadowline into an existing ceiling (which will usually have a ragged edge under the existing cornice) but I could be wrong...

Hi AW my photos are only two. The other four photos are from houzz. I search bedroom in Perth. As we still build with concrete slab and double brick walls over here in the west. Gradually changing to other build types. In WA, You have to specifically ask the builders over here to install skirting boards if you what them. They are not included in the normal build. I think the look is fine.

I have shadow line ceilings, mine are simple and inserted guessing mdf. The designer wanted to create more shadow lines. Keep it plain and simple. Also my ceiling, walls and skirts are all painted in Whisper white, ceiling in flat, walls Saturn and skirts in semi gloss. This was the designers idea, saying that pencil line ceilings always should be done like this.

Am the groove will require a wipe to keep clean as do skirting boards