I think you're pretty safe in rural areas with a simple lock to deter the opportunistic thief. More densely populated urban and touristy coastline areas where you have lots of bike traffic is a high risk though - that's where thieves like to go shopping with tools. I lost one of my favorite bikes in a big city when I ran inside for 10mins.... bike was locked with a Kryptonite U-lock. That turned me off bike touring for a while.
Now use a specialized ultra-compact rig for densely populated touring that is nearly the equivalent of a 2-item airline wheeled carry-on set-up. I usually just wheel everything with me inside (stores, restaurants, museums, tent, public transport, etc) and so have virtually eliminated my theft concerns.