#4: Sun Fading
We all love those gorgeous wood floors, but if you don't protect your floors from sunlight, they won't be gorgeous for long. In a single sunny season even brand new floors with UV protective finishes WILL fade if they are left in direct light. While floors are the most obvious victim of sun fading, don't forget about your furniture and the art on your walls. If you are a collector of original art, you definitely need to consider whether there is direct sunlight hitting any of your prized pieces. We may be cloudy for much of the year here in the Pacific Northwest, but when that sun comes out, it is not messing around, so don't underestimate it! If you have not lived in your home long enough to know where the sun will come in, consider the direction the window is facing: South facing windows can get sun all day, while truly North facing windows will never get direct light.
#5: Energy Efficiency
Of COURSE I had to get around to this one! Most window treatments, when closed, will increase the energy efficiency of your home. Some of the most common windows people leave uncovered are also some of the worst offenders in energy loss: Sliding doors and skylights. Skylights, in particular, leak out warm air all winter long, and on hot spells can really make a room uncomfortable. Other high up windows, like entry door transoms and stairway landing windows, can also be problematic in hot weather. If you are working on making your home more efficient, don't forget to cover these windows with something energy efficient, like honeycomb shades.
I left this one for last because I think it is quite subjective. If the window you are thinking of leaving off is separate from the rest of the room, like a kitchen sink window, or those little side-facing windows in a Craftsman living room, uniformity matters only if it matters to you. If you are worried about it, that may be an indicator that it will bother you if you don't do it! Pay attention to your own feelings (and those of your family), and let that guide you.