When others do it, is it frowned upon. When Sir Alex Ferguson does, it can be deemed a masterstroke. It is a rule of thumb that can appear unfair to less successful managers. Take the notion of selection. "Never change a winning team" was an old maxim. Ferguson invariably changes one, along with a losing team and a drawing team.
So entrenched is the idea of squad rotation at Old Trafford that it is hard to imagine a U-turn in the immediate future. In the process, the theory has gone from the sort of new-fangled and imported invention that the British habitually distrust - and one that foreign managers such as Claudio Ranieri and Rafael Benitez were heavily criticised for employing - to standard practice at clubs who anticipate playing 55 or 60 games a season.
When United won a third successive title as well as the Carling Cup, plus reaching the Champions League final in 2009, Ferguson displayed a remarkably sure touch in perming 11 from a cast approaching 30. In United's draw-riddled start to this season, a policy of constant change has led to regular irritation.
Gary Neville got his first taste of being an unused replacement on Wednesday since being an unexpected and unsuccessful choice at Everton in August. If such a selection perplexed back then, three more recent matches have seen wholesale changes with questionable results as Rangers, Sunderland and West Bromwich Albion held United. In each the charges against Ferguson are twofold: he underestimated the opposition and overestimated his fringe players.
Rangers' negativity should have been no surprise: a short spell as his assistant and a long friendship should have acquitted Ferguson with an understanding of Walter Smith's approach. In any case, the Rangers manager had joked about playing 4-6-0 (which may have planted an idea in Craig Levein's head). Yet a United side in which only Darren Fletcher retained his place ended with a back four consisting entirely of central defenders against a team with a marked reluctance to attack.
With Ji-Sung Park delivering an indifferent performance - a standard he has repeated all season - and Antonio Valencia breaking his leg, it was an occasion to illustrate that he cannot rotate with impunity on the flanks any more. Nani has become essential while Ryan Giggs, worryingly for a man who turns 37 next month and whose appearances must be rationed, is almost as important.
With Giggs injured, a consequence was that United dispensed with a left winger at Sunderland, starting with Michael Owen on the left and Federico Macheda in the middle of an ersatz front three. It required a rapid rethink. The teenage Italian seems a specialist impact substitute - a role in which he provided Javier Hernandez's winner in Valencia - while Owen's occasional important goals are interspersed with long periods of anonymity. An inability to play any role other than the second striker's is an ongoing issue.
Dimitar Berbatov's selfless shift at the Mestalla three days earlier may have accounted for his initial omission, but the failure of his deputies in attack meant he was introduced at half-time; as this was a game that preceded an international break and Berbatov no longer plays for Bulgaria, keeping the in-form forward in reserve seemed especially unnecessary. It is a ploy Ferguson sometimes uses, but the alternative and safer theory is to win the game and then rest players.
Moreover, Sunderland's prowess had been advertised. Defeating Manchester City and drawing with Arsenal provided ample evidence of their capabilities. It appeared to be overlooked. A confidence in his charges is essential for a manager, but a surfeit can be dangerous.
It appeared a problem again on Saturday. West Bromwich Albion's victory at the Emirates highlighted their form and skill. Yet Ferguson began without his first-choice central midfield pair of Fletcher and Paul Scholes who, following the international hiatus, should have been fresh. The second-string pairing of Anderson and Michael Carrick was disrupted further when Giggs was hurt, Darron Gibson brought on and the Brazilian shifted to the left flank. It left United with a notably lightweight duo during a spell when control of the centre of the pitch was ceded and an accomplished Albion side scored both of their goals.
The repercussions of Ferguson's deteriorating relationship with Wayne Rooney could extend beyond detrimental draws. United's No. 10 disputes the manager's theory he was injured when United faltered at the Stadium of Light and despite boasting some outstanding performances on the left in 2009, he was ignored when Giggs departed against West Brom, before being belatedly summoned in a quest for a winner. Perhaps, in trying to prove his point, Ferguson lost two. Out of form Rooney remains, but the alternatives are less enticing than they once were.
The loss of two proven match-winners - in Cristiano Ronaldo and Carlos Tevez - has been accompanied by a falling off in the displays of others, such as Jonny Evans, Park and Carrick, and lengthy absences of players like Owen Hargreaves, Valencia and Anderson. There is a contingent, including Owen, Gabriel Obertan, Bebe, Gibson, Macheda, Chris Smalling and Fabio da Silva, who are yet to convince they are bona fide United players; Hernandez and Rafael have displayed more promise without reaching that rank yet. This squad is an example that quantity is no guarantee of quality.
While Ferguson's tinkerings can contain moments of alchemy, such as the combination of the replacements Macheda and Hernandez in Valencia, the broader picture is that United are finding it harder to win even the supposedly simpler games without the majority of the core of Nemanja Vidic, Rio Ferdinand and Patrice Evra in defence, Giggs, Scholes, Fletcher and Nani in midfield and Berbatov up front. Instead, rotation has led to frustration and, with the title seeming ever more unlikely, it could bring recriminations.