I just run the fattest tire that will fit under the fenders.
Same. I started out, I think, with 32s. Tried 28s. Back to 32. Then 35. Then 38/40. 38/40 are a tight fit under the fender, so I may spring for new fenders at some point because so far, I have not regretted going wider. Although my touring bike will likely be limited to primarily pavement rides as I'm now in the process of switching to a Troll with 2.15 inch tires, maybe some 2.5" for mixed terrain touring. So, yeah, I say go as wide as you can within the limits of your current set up.