Hobbit on Quest (he/him)
It is an interesting principle and one I largely follow - but I also subordinate it to being a fan of the PCs. Example: If the PCs try to scry on one of their enemies, they get a save to resist. Great. I roll their save and go with the results (in one case in play, the save succeeded, scry failed). But if the PCs productively work the situation for a couple of days to undermine their target's save, then when they try to scry again, even if the target gets a save.... screw that save. It fails no matter what I roll. My devotion to giving the monsters an even break is secondary to being a fan of the PCs and how they work cleverly to achieve success.Fairness, on the DM's part, extends into the fiction as well.
Gygax emphasizes his point of "Always give a monster an even break", and he's right: a DM has to be fair to the opponents just as she has to be fair to the PCs.
Further, if a DM is fair to the PCs in the fiction then by extension she almost can't help but be fair to the players at the table.