Many robot enthusiasts discuss whether to double up the ankle servos. Afterall it is the lowest part of the robot body and experience most weight. However I believe the hip servos naturally keep the legs from spreading and so should not require the ankles to do all these works. Afterall human ankles are very skinny and have little muscle.
In the below video experiment I unscrew the servos to allow the hings move freely. To prevent the robot from falling sideway the amount of movment is limited by lowering battery down so the ankle cannot rotate up. As you can see the basic movements i.e. front/back walking, side shifting and turning is uneffected. It is wobbly because there is only one screw holding the foot to the leg. I think I can still manage it to kick football this way but of course prolong single leg stand is not possible.
What this mean is no you don't need to put strong and expensive servos there. In fact for my Knight Templar II, I will remove the ankles entirely because it is currently too top heavy and I need a stronger arm servos. So this can make the robot lighter, with lower CG and allow it to have stronger arms.