However, we must always remember that just because people appear safe now does not mean they will be safe in the future.
Very true; we only need to look at how plenty of Republicans who were "safe" in 2002 and 2004 lost in 2006 or last week. Or at 1994. Still, I think you can generally see certain signs of possible vulnerability under the surface (beyond "Presidential candidate x did badly there" and so on), though the way House elections work can make it harder than it might otherwise be.
The amusing part is that the district in AR that seems least likely to fall in a hypothetical midterm nightmare is the one that voted for a Republican Rep as recently as 1998...