It's recommended to keep arms and legs straight to avoid harming the TPE/silicone or the skeleton.
Provide the doll's got standing feet*, you can stand a doll leaning against a wall with a piece of foam behind her back, this stops her marking the wall, protects he back from the wall and the fact the her ass is away from the wall it wont go flat.
In bed or in a horizontal position they are best returned to a straight position
'If' they ever are going to there in the same position for a long period of time, the chances are they not going to be in the same positions for long, so they might just need legs opening and closing when require.
*Dolls with limited standing means it might wreak their feet if they are standing around for a long time.
