For those of us doing the "standard" meditation practice of sitting upright in silence with our eyes closed to observe ourselves, it's good to be aware of the position we take to do this. I've always personally been rather inflexible through my years, so I generally take the easy way out and allow my back to be supported by something as I meditate. I reckon that this is much better than not meditating at all.
That said, I do find it important to occasionally try stricter postures, and of course if I'm not meditating at home, it's a good opportunity to try different things as well.
When I'm on the floor, I pretty much always have something keeping my butt off of the ground to relieve tension in my legs. Sometimes, I might even run a pillow across the underside of my knees to relieve tension there as well. My goal is to allow me to focus on my meditation instead of the pain of sitting awkwardly.
Read some tips about Meditation Posture
Watch the Breath
Articles on mindfulness meditation.