Bookcase sides thickness

[UncleFestool]

I've got the rough dimensions and design for a built-in bookcase with low level cabinets under. Construction will be 3 bookcases just under 700mm width screwed together to make construction easier. I was just going to go for 18mm MDF for all sides (25mm shelves), but thinking it may be better to go for 9 or 12mm sides where they double up.
It will all be behind trim pieces/fake face frame so visually the thickness isn't that much of an issue. Is it worth going with 18mm for any reason other than keeping it simple?

Same with end panels/cheeks, was originally thinking 18mm for those but probably overkill. Your thoughts?

 

[niall Y]

When building bookcases in the past, I have used thinner sides where they abut each other. But this was to maintain a uniform look to the uprights. So if the thickness in question was 25 mm the abutting sides would be 12 mm. This is only really needed if you can see the uprights. But if you are using a covering frame, I would keep things simple and use 18 mm throughout.
Thinner sides can be an issue, if you are using adjustable shelves, as you do need to have some thickness to enable you to fasten Tonks-strip or Banjos, etc.

 

[TRITON]

In solid oak, black walnut etc we used 25mm. Basically as inch boards thicknessed down to a basic starting point of about 25/26mm and nobody saw the point of turning any more into woodshavings. In faced mdf the standard (double faced) at 19mm.
Keep in mind you'll likely have dados in them to house shelves, so thick is better than thin.