What waterproofing rating is sufficient for a navigation device for touring?
Easy bit - Touch screens don't usually work when wet, you can easily test it, sprinkle some water on the screen and see how it goes. 

Because phones are high volume consumer goods designed as life style toys, not outdoor equipment, as far as IP ratings, I personally believe the claimed IP ratings are based on marketing need for a bigger number than the other guy, tempered by engineering squealing that IP99 is not a real rating, and Finance insisting at that price point they have a budget for X returns out of Y sales. The exception are devices sold as ruggedised.  Read the warranty fine print - I bet it excludes water damage.  

IP67 is 1 meter for 30 minutes, but the test is done in factory conditions on a new phone, not after they are a few years olds and lost elasticity, and the user has contaminated the seals with dirt and dust. I had an IP67 phone with a single o-ring seal for the battery cover. I was happy to get it wet in the rain, but would not have left it on handle bars for hours in driving heavy rain while riding. Driven rain on a fast down hill could exceed the pressure of 1 meter of water.

I would also be concerned about "solar driven" moisture entering the phone. It gets wet, sun comes out and shines though the water onto the phone. The water close to the phone surface heats and vapor pressure increases, driving water vapour (not water droplets) into the phone. The expanding and contracting of air inside the phone from changing temperatures can also cause a negative pressure that sucks water in. 

Although the risk is small, if you plan on riding with the phone on the bars in all weather, put it in a case or accept you might need to buy a new phone. But in a case, the touch screen may or may not work - you would need to test it.