Both safety positions are expected to cover in today's NFL, so the majority of coordinators are going to call their safeties interchangeable. Sometimes that will mean a player will play only on the left side of the field and not switch to the strength of the offensive formation. Sometimes they'll switch but both play coverage first, such that either will come up first based on run-pass reads.
Teams that I think will use the SS in a run support role most frequently might include the Cardinals, Saints and Raiders. Beyond that, there might be an argument that every other team in the league will either be using both safeties in coverage enough to consider them interchangeable, or bring the "FS" up into the box as often as the SS -- like the Giants, Eagles and Rams seem likely to do.
We've been pushing the skill set over positional designation when scouting secondary players for some time. Getting some indication of how often a player might be tasked with run support first helps (see the Saints notes on Roman Harper this offseason), but erring on the side of the player with better run support ability and ball skills in coverage has been the better way to go over the past few seasons. It's a reasonably safe bet for the FS to be less valuable than the SS still, because even the interchangeable teams tend to put the bigger, more physical player to the strong side, but it's far from the safe default it used to be.
