When should you use a carpet runner for your steps? First, let me clarify what a carpet runner is - it's when you install carpet on the steps, but you leave some space at the ends so you can see the ends of the hardwood (see below).
So why do people add runners? There are 3 main reasons: 1) decoration 2) safety and, 3) reduce creaking.
1. Using a carpet runner for decoration. If these are your front steps, and especially if they can be seen from your entryway, it is one of the first things your guests (or potential buyers) see. So, if you want to make a statement and have a grand entrance, this is a really easy way to spice to things up. It adds some flair and sophistication, and nicer houses tend to have these. Runners can be very formal in design (think classical patterns or oriental rugs) or they can be simple. Personally, I love this one because it's pretty and formalizes the entry a bit, but not too much. It had multiple colors for design flair, but not too much, so it doesn't compete with the oriental rug my customer has in her living room. I also love the color since it's elegant and contrasts well with the hardwood. You prob can't see this well from the pciture, but the dots in here have a highlight color in here that matches the butterscotch color wood and a low light that gives it some depth.
2. Using a step runner for safety. My 2 most frequent requests for this circulate around babies/toddlers and aging pets. For the new mom who needs to carry her newborn up and down the steps, she may feel unsafe or afraid that she is going to slip with her most precious possession of all. Likewise, as toddlers are learning, growing and trying to get up and down the steps, they often slip. The runner make it less likely for them to slip and/or when they do, it cushions the blow (I suppose the diapers help too). I also have a lot of customers who love their pets so dearly and as they've aged, the pets are having challenges with the steps. Carpet runners can provide good solutions for them as well.
3. Using a carpet runner to reduce creaking. Adding a carpet runner will not fix the creaking. But, it will usually muffle the sound. Sometimes the creaking is due to poor installation of treads or sometimes it's just because the house is old and things have settled. Other times, it's because there are no treads (treads are one big piece of wood) and an amateur did it the cheap way...they just added in pieces of wood (the same type you would use on the floor) and added a stairnose to the ends. Over time, this cause the wood to rub against each other and hence the creaking. If you do have this, you could partially address this by adding some screws in the steps to secure the boards (it won't solve it and it may come back over time, but it will definitely help). You can then put the runner on top to both cover the screws and further reduce the noise over time.
Now, how do you handle carpet runners on steps if you are selling a home?
This depends if your client currently has a runner or not.
If your customer has a carpet runner, it depends on what condition it's in (is it old, dirty, coming apart?) and what it looks like (is it neutral or taste specific?). If it's in bad condition, something has to be done, ESPECIALLY if it's by the front entrance. This is the buyer's first impression, and you want it to be a good one. So, either replace it (and replace with something neutral) or better yet, just rip it up and sand & refinish the steps - that will prob. cost less and be more neutral).
If they don't have a runner, the simpler answer is to just leave it as is, and if new buyer wants to install one and pick their own style and colors they can. Sometimes, in some places adding a runner can really formalize the place and help sell it. But, if you do a nice runner, you don't always appeal to all tastes and some customers prefer to either change or not have any runner, so then their perception is that they will have to pay to remove or replace it.