I have the same issue with my servos twitching when I apply power to them. I tried adding a pulldown, but that didn't do anything.
I did a test just now to see what would happen if I applied power to my servos without the Arduino supplying position data though, and the result was interesting.
If I disconnect and reconnect the power, the servos twitch. In fact, not only do they twitch... one of them will actually rotate a significant amount if I apply and remove power over and over. The other will remain in place and simply twitch. (Same brand, same type of servo.)
It may be when starting up the twitch is due to there being no signal for the servos. I'm not sure of any good fix for this though. One would have to switch the power to the servos on after one's sketch is running I suppose.
But... I just did another test with the sketch running and I applied power to the servos, and they still twitch. Though they don't lose position. And they seem to twitch less.
Anyway... it seems possible there may be no software solution for servo twitching on power up. And there may be no easy hardware fix either.