Just finished a four year stint on the 767. No restriction on the a/t on/off when in manual flight. Personal preference. When the wx is gusty or it is windy, the a/t tends to be a lot more agressive than the pilot would be him/herself. The a/t reacts late in speed reduction, and reacts agressively when speed is low. And these large thrust inputs have a large impact on pitch, this increases workload. So from a passenger comfort point of view I tended to fly a/p-off, also a/t-off. But there was no limitation from our fcom.
Now on the 777, you need a good reason to turn the a/t-off. The thrust vector has been filtered out with the fly-by-wire system in normal mode. So thrust changes have no impact on a/c pitch. Very nice, the 777 is a 767 that has gone to finishing school. Both great a/c, but the 777 is the cat's whiskers.
Greetings O.