I actually think it should be a sliding scale. At low levels, I don't mind the CRs being tuned to low optimization, because that's where new players start, and at those levels things are so swingy that high optimization can still get challenged.
At mid levels, moderate optimization. I do think the current CRs are too easy in general and could use a look at.
At high levels, assume strong optimization. Not because all high level players are minmaxers, but because there are SO many crazy combos at high levels you should just assume the party will have some crazy track up their sleeves. Further, for high level parties death isn't really that big a deal, high levels parties have plenty of ways to escape bad fights or just raise dead as a last resort. So high level challenges should be REALLY challenging.
At mid levels, moderate optimization. I do think the current CRs are too easy in general and could use a look at.
At high levels, assume strong optimization. Not because all high level players are minmaxers, but because there are SO many crazy combos at high levels you should just assume the party will have some crazy track up their sleeves. Further, for high level parties death isn't really that big a deal, high levels parties have plenty of ways to escape bad fights or just raise dead as a last resort. So high level challenges should be REALLY challenging.