Filler strips are generally required in a corner situation if the corner is 90 degrees and there isn't a corner cabinet involved. Other than that, you do not need a filler next to appliances. Trying to line up cabinets top to bottom is often not very critical. It is difficult to see all of that unless you are in a specific location in the kitchen and are paying attention to that issue. The fewer fillers, the better.

You don't always need fillers between a cabinet and the wall. It really just depends on how square the wall is. That said, filler can definitely cover up any issues with that. You also don't need filler next to an appliance, unless it's needed to line up upper and lowers for some reason. For instance, the upper run to the right of my range was 39 inches. The lower was 38 inches. We used one inch of filler next to the range to make those two match so that our range hood would be centered perfectly over the range. You should try to minimize filler as much as possible, but can certainly use it in those sort of situations.

Filler is always required against walls that cabinets butt into. But, it's the cover panels thickness that most people forget to account for, and forget to use in the appropriate locations.

You also need filler if a drawer needs to clear an adjacent knob/pull, or doorway casing. Or if a cabinet door needs to open toward a wall, I think.