One thing that really helps my meditation is being alert and comfortable while I’m meditating. To do that we do not need to sit on the floor or even sit cross-legged. I guess it’s true that if you grew up sitting cross-legged, or just have great knees, then that will work well for you. But lots of us in the West didn’t grow up that way. And we may spend many hours at a desk each day or standing up all day long and our legs get tired. So when we meditate we need to find the posture that works best. If I sit cross-legged my legs may go to sleep. So I sit often in a chair with feet flat on the floor. That way my back is straight and I stay alert throughout my meditation session.
And sometimes I do sit on a low, wide chair and in a cross-legged position, which I adjust during my meditation. I think that is OK to adjust our position, and even sometimes I’ll get up and stretch a bit and walk around before sitting back down.
This is all to say there are really no hard and fast rules about posture and form, but that we need to be sensitive to who we are today. Is it a “chair” day or a “cross-legged” day or something different?