Okay, maybe I am biased about this. But I would rather recommend to do the photo that involves maybe getting the dress dirty than playing it safe and never risk anything. Here is why. Most wedding dresses have layers. So if you walk over some mud or dirty leaves on the ground, the outer layer that people actually look at stays clean (most of the time).
The best nature locations are never the cleanest
If you go to any kind of nature or park setting on your wedding day, there is a high chance that you can’t always stay on a path. Also, standing on asphalt doesn't really look like you are in nature. Of course this depends on the location. Sometimes it works to just show your upper body in the photos with the surroundings.
Don’t get me wrong, I am not suggesting you should do crazy things on your wedding day just to get the photo. I won’t ask you to do anything you are not comfortable with. You can also make yourself more comfortable for example by bringing flip flops to walk between spots. The couple photos are supposed to be fun for mostly. Even if I create the best photo ever, if you hated the situation you were in when I took it, that feeling will come back when you look at the photo.
All I am trying to say is that there is no need to worry too much about your dress. Take a look at the photos below, this bride walked barefoot through the woods, jumped with her husband from one rock to the other and walked through some muddy paths to get these beautiful photos. And we had fun doing it, because she didn't worry about it at all!