When a cabinet side is up against a wall, you want just a little filler/spacer so that drawers don't catch on variations in the wall. (Watch out for doorway frames as well.) Because they are frameless, the drawers and doors use the entire width of the cabinet. If you have a door that opens against a wall, the handle will hit the wall before the door is completely open so you may want filler at least the depth of your handle.
Most often, plinth/toekick material is used as filler. Some people don't like the finish as well so they use pieces of cover panel. If you do a search for threads with "filler" in the title, you'll find some discussions. Here's one: Filler - how to scribe and install