It's not really a straight-forward question though is it.
At what stage of a cup competition does fielding our strongest team start to pose a risk to our PL survival? For example, at this stage we have little idea how well we are doing in the PL survival stakes. By the fourth round of the FA...
Assuming we go out of both cups at the first time of asking, then we'll have played 40 matches this season.
If we were to win both cups, we'd have to play 50 times.
Personally, I think that players want to play in every game. Play them. Or at least play a slightly stronger squad than that...