It really depends on the talent gap between your starters and your backup. If your starter and backup are fairly close in terms of talent, then fairly fresh is best. If your starter is significantly better than your backup, then somewhere along the lines of getting tired (or in extreme cases, where your backup is terrible, tired could even work) for your starter, fairly fresh for your backup.
In real world example, compare usage of Jeremy Lin to other teams. Lin has pretty much been ran to the ground night in night out, to the tune of 40+min in multiple games. Obviously, D'Antoni feels that a tired Lin is better than a fresh Mike Bibby (or whichever other point guard they have). Talent gap (as of now) is huge between the starting PG for the Knicks and the backup. Meanwhile, other teams with more quality backups would have minutes distributed more evenly.