Stop leak products can cause problems. Stop leak products work in 2 ways:
- Particulates: basically grit that will get caught in small holes to plug them
- Chemicals to soften seals: Old seals lose flexibility, the idea is that they "rejuvenate" old seals, making them more flexible so they do their job longer
Particulates are abrasive, and having abrasives in a system with pumps will cause damage long term. Not all stop leak products have them, so that may not be a concern. You don't want to soften seals that are already doing their job as you could end up shortening their life. The chemicals also work on other "organics" like hoses, shortening their lives as well.
If you don't have a leak in your power steering don't use a stop leak product. Use plain old power steering fluid.