Considering how quickly the Eagles deal happened, I think it's obvious that the Phins wouldn't have moved out of #3 if they didn't already have that deal in place to go back up. So I don't think staying at #12 was ever an option.
But for the sake of argument, if you wanted one of the top 4 receivers, then yes it was absolutely the right choice. The Eagles, Lions, Panthers, and Giants were all potential landing spots for the last two receivers once Pitts and Chase were off the board. Staying at 12 nearly guarantees you miss out. You can't look at the picks now and say, "oh look, the Lions and Panthers didn't go receiver, so the Phins could've just traded up over the Giants." The Lions and Panthers COULD have taken a receiver at their spots. Grier can't know what their boards looked like.
Even with the hindsight of how to board fell, you can't assume it would've gone the same way if the Phins stayed at #12. Maybe the Lions or Panthers had Waddle graded higher than Sewell or Horn, and so if the Eagles take Smith at #6, maybe one of them takes Waddle. Or maybe if the Rodgers news doesn't break, the Broncos take a QB. Then the Cowboys don't want to trade out because they get their target in Surtain. Then the Giants take the last receiver right in front of us.
The only guarantee going in was that the top three picks were QBs. So if you wanted one of the top four receivers, you needed to be at #6 or #7 to guarantee one of them.