We were also in Yellowstone last week and did many Earth and Virtual caches throughout the park as well as physical caches outside the park. I had read that cell phone service was pretty spotty at best before our trip. I took quite a bit of time creating an "offline" list. You can do this with a pocket query, but I also used GSAK to make it a little better. I used the offline list on both my Android Geocaching app as well as C:Geo. I also had a print out with a Garmin GPSr just in case, but didn't need it. My GPS on my HTC One worked great even without cell phone service. The offline list in C:Geo was a lifesaver and worked flawlessly. I have definitely learned to make lists if I'm going on a trip. It makes things work so much better and cuts down on a lot of frustration. You lose cell phone service even in the counties of NC where I live, so I'll definitely be using lists more often.
Just thought some would like to hear how we tackled it. My list contained 73 earth/virtual/traditional caches and we found 59 throughout Colorado, Wyoming, Idaho, Montana, and the "Welcome to Nebraska" just to get another state.