Destinations can be sprung? How exactly? Mission diversions are always optional. I can't recall a single time i was forced to travel a long distance.
Why not suggest people avoid those systems if they don't like the travel time? There are always other options. No need to avoid surrounding systems, that would be near impossible since within 20LY range there is usually at least one or more systems where you might get missions with long travel times. Just don't take those.
There are a few mission types (planetary scan, sensitive X retrieval, wetwork, etc) where the destination will be a mission signal source, so you don't know in advance how far out it will be. Those mission destinations can generally be at any distance from arrival out to the "edge" of the system, which seems to be loosely defined by the distance to the most remote body. But again, that's exactly why you learn which systems in your working area have those distant secondaries.