(https://2.bp.blogspot.com/-t7zA8XH_VuM/W6VF17M9yfI/AAAAAAAALx0/WfCvoUlqtlYtYfsBO7hlFgG6vxP59PxqQCLcBGAs/s1600/2018-09-22-PrintableSceneryRuinsInternalBase.jpg)
I added an internal base to a complete building assembly, and as I suspected it does look much better. If I add bases to the floating corner pieces, they'll need to be fairly minimal so that they don't interfere with each other when laid out on the table. I should be able to get away with ragged triangles, I think.
It does add considerably to the preparation time, but it doesn't need to be done at the same time as the painting of the shell. I can do the basing progressively as I have the time.