I’m of the thinking that you play them for a series or two in at least two of the games. If in the first game they are just clicking, maybe you sit them the rest of pre-season. If however, their timing is off and they don’t seem to be hitting against an actual opposing team, you need to get them ready for the games that count. With only 16 games, they all count. Well... unless you have nothing to gain at the end of the season. I still don’t think sitting the starters was a bad play last season for week 17.
That was a tough call, and I don't know the answer. I think the coach has the best pulse on the team, whether to play them or not.
Sitting players, is kind of like prevent defense. It can be iffy, but it is also hindsight ripe.
McVay seems to be pretty clear on the protecting players, which seems a little unusual, if only for the fact that he is such a young coach. I think something in your career, probably influences coaches in this area of decision. I've seen it go both ways, but I have no problem with it. I'd be pissed if one of our key players got hurt before the season started, or right before the post season, I know that.