Know what these two images have in common? No, it's not the subway tile.
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Each project is different and has different circumstances so there are no hard and fast rules except one, you don't want them to stand out or contrast with your material. You don't want outlets and switches to take the thunder away from the star, your beautiful backsplash.
These dark outlets really distract from the pattern of the backsplash in this kitchen. A light neutral would have helped hide them.
The white outlets here interrupt the pattern and have a high contrast with the gray tile. Your eye goes right to the bright white spot. A gray color and a horizontal mounting at the base of the splash would have been a good solution.
Simple white outlets on this white tile would have blended in.
Lutron controls have a great color selection to choose from. They are somewhat more costly than standard outlets and switches, but you can really get a good match and a very sophisticated look for your backsplash.
If you are going to use your outlets a lot on that wall or leave items plugged in, the trailing cord can be more unsightly than the outlet. It's also a pain to bend down and look under the cabinet to see where the outlet is every time you want to plug something in.
Here is a standard light almond color outlet on a neutral tumbled limestone splash. It blends in really well.
The outlet above is mounted horizontally and blends with the backsplash colors. This is a Lutron greige color and is as inconspicuous as possible.
The outlets here are mounted horizontally and fit within the size of the tile and just underneath the inset. The expensive handmade glass tile mosaic is uninterrupted. This takes careful planning when locating the outlets on the wall.
A beautiful kitchen with unobtrusive outlets and switches. You notice the marble tile, not the white switch plates. 
This red tile doesn't relate to anything else in this space. It looks out of place and maybe like the homeowner added it at the end of the job.