Personally, if I believe I can estimate it to the closest meter (which means it's pretty short or it's next to an object of known, similar height) I'll go ahead and add the integer height -- always with no decimal point, to indicate the lack of precision. Failing that, I'll just add a `building:levels=*` tag. Whether that's any use to 3D mapping, I dunno, certainly the renderers could use it as a rough height estimate if they were so inclined. But if you're doing complex things like roof shapes, it's probably no help.

Personally, if I believe I can estimate it to the closest meter (which means it's pretty short or it's next to an object of known, similar height) I'll go ahead and add the integer height -- always with no decimal point, to indicate the lack of precision. Failing that, I'll just add a `building:levels=*` tag.