When meditating, the fundamental rule of posture is that you be both relaxed and alert.
Some people find it best to sit in a straight-back chair.
Others find kneeling on a prayer-bench best for them. Still others choose to meditate sitting on the floor or on a firm cushion. Once you find the posture that you prefer, stay with that same posture each time you meditate. Back straight, feet flat on the floor. Back straight, hands comfortably on your lap.