White risers look beautiful, but, they are also a high maintenance. They get spots on them, where a shoe touches them and if there are children living in the house, you will be scrubbing and repainting them often. I would leave them alone. They can be washed off easily right now.

I agree with cat_ky. This was done in a condo, and although the risers looked great, it did not last, due to people refusing to remove their shoes. Additionally, staircases lacking runners or similar are lawsuits waiting to happen. Dependent upon the ages of your home's inhabitants/visitors, you may also consider installing a handrail on both sides of this staircase. Check your local codes first, because many areas now prohibit installing a straight run handrail. They now require a "turn" at the top and bottom, so no one catches their clothing/handbag on the staight rail.

I've had white risers on my stairs and never any issues with marks. Sounds more like a issue with the paint that was used. Mine are also bare wood and not an issue, carpeting or runners is more likely to cause a fall.