Firstly, there are always exceptions to the rule but there is definitely one thing that you don't want to do when hanging art and that is to hang it TOO HIGH. It doesn't matter how tall your ceilings, husband, or wife are, I hate this excuse.
There are several ways to hang art above your sofa. Having the right piece or pieces of art is going to be the harder thing to accomplish. We want to create balance with your sofa and the pieces above it. If you have a large piece it should be 2/3 of the width of your furniture. One piece can be tricky to find so you may want to consider a ledge to set multiple pieces on or multiple pieces that can balance the space. You will want to hang the art 8"-12" above the back of your furniture.
What is the appropriate height?
There's no straightforward answer unfortunately.
Art hanging on a simple wall should be hung at "eye level" - this won't be everyone's eye level but we will refer to this in general. Eye level is 57"-60" to the CENTRE of the painting. Some say 58"-62", and that is fine too. A couple of inches here or there isn't that noticeable - that's what she said ;-)
Above a CouchThere are several ways to hang art above your sofa. Having the right piece or pieces of art is going to be the harder thing to accomplish. We want to create balance with your sofa and the pieces above it. If you have a large piece it should be 2/3 of the width of your furniture. One piece can be tricky to find so you may want to consider a ledge to set multiple pieces on or multiple pieces that can balance the space. You will want to hang the art 8"-12" above the back of your furniture.
I like the idea of a ledge, just ensure the width is right. It's easy to change up the pieces and add textured bits (like this tray & the plant) to the mix. Hit the thrift stores, Homesense, local art shows, holiday markets....
This triptych could be hung a wee bit lower, but the balance of the art with the couch is right. You can't see the ceiling height here but there should be less space between the back of the sofa and the bottom of the art versus the top or the art and the ceiling.
Love this placement and balance. Chef's kiss.
A narrow space by a door or in a bathroom is a great opportunity to stack art. Just remember that when you stack your art the same 58"-62" rule applies. You can hang multiple pieces of art above and below each other but have the centre of the stack at the 60" (ish) spot.
This is a beautiful example of stacking art AND placing art at a height appropriate for the use of the space. I would argue that one can hang art quite a bit lower in a dining room or in a space where you are sitting so that you can appreciate the art while you are seated.
Above the Toilet
This is a very popular place to hang art! What a better place to have a unique piece for you and your guests to enjoy. There are several ways to hang art above the loo. Stacking is fantastic in this spot. A long narrow piece may fit here too. One statement piece could also fill the space. Here you want to judge the piece with the space. A long narrow piece of art may require you to hang it 4"-6" above the top of the tank. Other pieces can be hung using the 58"-62" to centre rule. Have fun here!
Whether you choose to use a mirror, a piece of art, or multiple pieces of art above your mantle please consider keeping it low. Some choose to lean art with it rested right on the mantle. Even if you want to add candlesticks or other little pieces of decor you can have them overlap the bottom of the art.
Don't get me started on gallery walls. Ugh, they scare the crap out of me (while we're on the toilet topic). Apartment Therapy has some great tips and there are a lot of examples of gallery walls, I'll let you go down that rabbit hole on your own. If I were to attempt a gallery wall I would break out the painter's tape and start mapping out the wall before I hammered one single nail into the wall. Mix and match art, with prints, and mirrors for the best look.
Good luck & happy decorating!