Every layout and every structure is different, so there is no hard and fast rule to go by. Most all of the roads on my layout are blacktop roads, which I make by cutting appropriate width strips from roofing felt, otherwise known as "tarpaper", which isn't very thick. Many of my structures are DPM or Smalltown USA structures which do not have sidewalks included; I add them and make them from Evergreen Styrene sidewalk sheets. If I butt the sidewalks up to the front of the structure, I do not raise the structure, If I tuck them under the front of the building, I use .040" x .060" strips of styrene around the perimeter of the building. The sidewalk sheets are .040" thick, some kit sidewalks (bases) are thicker. I do rest the front edge on top of the road to give the illusion of a raised sidewalk in most cases.
A lot is also based on personal preference. Again, this is what I do, there are many other factors to be considered.
Willie