You may want a rug pad to keep rugs from slipping, especially if you got an itty bitty rug for a high traffic area. In these scenarios you'll want something made out of natural rubber that won't fall apart over time.
If you are a frequent rug sitter, you may want a felt rug pad to add a little more cushion.
And if you are super fancy, you can opt for the combo felt and rubber rug pad.
While there are scary stories on the internet about latex backed rugs ruining hardwood floors, we've only been able to find two documented instances of this and they were both seagrass rugs. If you are the type of person who never jaywalks, eats ice cream slowly to avoid brain freeze and tells their doctor the truth about how often they take their vitamins, then you should definitely get a rug pad.
Most rug pads are pretty easy to cut, so if you created a custom size for your space, you can always buy a bigger sized pad and cut it down to the size you need.