I think any unwritten rules in sports, particularly the ones that affect the outcome of a game, should only apply to the team that's blowing the other team out. The definition of a "blow out" obviously is subjective but there's usually a point where the outcome of the game has been determined. In baseball, you stop trying to steal bases. In hockey, you don't put out your top guys if you have a powerplay. In football, you stop passing. Etc, etc, etc.
But considering the amount players are being paid today to do whatever they can to contribute to their team winning and the revenues that are at stake from fans expecting their team to do whatever it takes to win, hitters should do whatever it takes to get on base. The games today are different than they were back when the unwritten rules were "written".