I think it's true to say that players would not intentionally play badly. I think there are scenarios where teams would (in some ways) take their foot off the gass though. Remember when we played Hucknall away on the last day of the 2004/05 season? As news filtered through that Harrogate were getting spanked by Southport, we substituted key players in order to rest them for the playoffs and went on to lose 5-2 in what had been a close-fought game.
Teams who are guaranteed a playoff spot are likely to want to preserve the energy of key players and avoid unnecessary bookings by subbing them or not selecting them at all. I don't really blame them for doing that to be honest as it makes perfect sense.
There is a clear difference here between players lacking integrity and managers using a bit of common sense.