When I play, I'm actually focusing my eyes on the bridge markers, not on the strings themselves. If you try to focus on the strings, it's easy to get confused between the string you're trying to hit and the one that runs underneath it, coming from the other bridge. I find that the bridge markers are a much more friendly way of helping me aim my hammers!
There are many schools of thought about how often to change hammered dulcimer strings, and our answer to that questions sometimes surprises people. If you have phosphor bronze strings or wound strings on your instrument, it's not a bad idea to change those every few years. They can start to sound dull after a while. But we really don't believe you need to change your plain steel strings ever. They have a protective tin coating and you're not playing them with oily fingers, so unless you scrub the coating off with steel wool and the strings get rusty, there's really nothing that's degrading their tone. It's hard to believe when you're used to a fingered string instrument like guitar or violin, but think about how often people change all their piano strings. Not often, right?