As Sinanerdem says, setting take off thrust is simply ensuring the a/t increases the thrust to that set in the FMC. And remember at around 80 kts (84 I think), the a/t will enter arm mode, so if any manual tweaking of the thrust is required, it can be done, without the a/t returning to its previous setting. If the a/t is working correctly though it should be at the correct thrust way before this.