I used to do the same thing, like, sit on a cosy chair, lean forward, arms on knees, legs a bit open and flourish. What I'll say is that its good for practicing new cuts which you are unfamiliar with, somehow, because having your elbows on your knees for support really prevents your hands from moving apart from each other and splaying cards all over the place. However, if you are intending to go out onto the streets and flourish, or in future, stand up and flourish (like what I'm progressing to), you may face a bit of a problem. I have a friend who can do a good-paced Sybil sitting down but it gets progressively harder for him when he stands up. Somehow the combination of height with a lack of support for the elbows when standing up makes flourishing standing up slightly different, so if you plan to do that in future it might be good to start early. Oh, and Cardmagic is right, bending over for a long period of time can screw your back. A bit is fine, not too much.
