I think as long as you can get fat enough tires and sit on a sprung saddle that any frame will suit you just fine for washboard roads.

An upright stance may serve you better if you are doing more technical stuff but you can still ride singletrack on a touring bike no prob and will be faster on the roads-While touring, the road feel of my LHT reminds me a lot of touring in the 80's on rigid mountain bikes, athough the handlebars and stance are different.