Well you did say clutch abilities were due to the hitter's increased focus in key situations.
I don't think anyone is going to crucify long retired players like Farmer and Jackson if they said they took a few innings or at bats off. Certainly not at this point in their lives.
I will say that I do believe players always try regardless. And there are obvious reasons. Great hitters just don't let up. It is part of what makes them great and statistics bear it out. Everyone else just doesn't want to get caught from behind by the bench player or the AAA player or the "next big thing" in the press. If someone is hitting .218, noi one says "he's hitting .218, but a lot of those at bats were unimportant so he probably didn't try." They say "he's hitting .218. Period. And if the guy who has his position in AAA is hitting .318, the pressure to keep the job is on. Ask Coco Crisp about this.
And it's really not even limited to players like Crisp. How many times did we hear about Steve Lyons taking over for Wade Boggs? Heck, sometime as georom who he thinks should be starting at second base for the Sox this year - Dustin Pedroia or Brock Holt? How are those twi even comparable? But apparently, the younger, cheaper, healthier (and far, far worse) Holt looks better to some.
The bottom line is when your job depends on performing, you have to keep the intensity up all the time. If you don't, someone else will...