Placing Structures

The list of possibilities is rich; here are just a few: - Shrubs. Lots and lots of shrubs. - Use white bathroom caulk, then sprinkle the remainder of the layout liberally with scale snow. - Bank the base of the building with sand, and make the layout a desert locale. - Continue building them crooked, but touch them up on a belt sander before installation. - Continue building them crooked, but before installation, draw around the base with a pencil, then rout inside the line enough to inset the building into the layout deeper than any potential gap. - Put tee nuts into the corners, and tell visitors it's an earthquake-proof building design, in common use in the area you're modeling - OR, strangest of all - - build them straight, on a flat surface, because it's ultimately less cumbersome.

I just place the building and then use whatever scenic material "fits" the location.
Dirt, weeds, junk, rocks, pavement, grass, bushes, flower garden, vehicle....

I make a fine "sausage" of modellers clay (non-drying) and lay it along the bottom edge of the building before placing the building on the layout. The little bit that squishes out can be flattened/feathered with an old screwdriver. Modellers clay like "DAS" can be got in a variety of colours quite cheaply.
Just cut some pieces of card to right angles, glue to your flat surface and assemble the adjoining walls against them.

You need to build a foundation for the buildings and that goes onto the general scenery to fill in the holes.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!