The appxmanifest has entries for developers to fill in four different tile resolution images for each kind of tile (small, medium, wide, large), the different resolutions are: scale-180, scale-140, scale-100, scale-80.
From what I can tell if the developer fills in the largest resolution image (scale-180) the system will automatically scale it down when needed on lower resolution displays, thus it works everywhere. So do most developers really even need to bother filling in all the different tile resolution images? Seems like it will just bloat the size of the application for nothing.
My question about filling in the different resolution images not about whether I need to have the small, medium, wide and large images.
It's certainly not necessary, but it is recommended. Scaling images down (or up) can cause visual artifacts that may not be present in the original image. It may be the case that a particular image looks fine in all scaling plateaus, but others may not (this is especially true for bitmap images). Again, it is up to your discretion, but worth visual inspection at each scaling level.
MSDN has a scaling guidelines document that could be helpful.