I'm the same as Christopher T. I practice standing because I need to know my angles and height at which props and cards need to be held for proper viewing. One thing to keep in mind is your angles when you approach a table and they are seated and you are standing. Think Invisible Deck and you'll understand.
I stand up. When you sit down the spectator is usually also sitting. If you sit you will probably be at a table keeping your hands at the same level as you chest creating a revealing angle that could accidently peak. You are also in more control of the audience when standing while preforming the trick.
The last year or so I have focused on practicing sitting down. I have had an increase of venue situations that I have been joining the spectators at theirs tables. Never a bad idea to be versatile.
I will practice standing in front of my mirror or I might practice moves and slights sitting down while I listen to music or watch a movie. I always perform standing up.
I used to practice sitting until I tried performing sitting, but basically the spectator saw everything I did. I learned that standing gives you a lot better angle control and sitting, not so much. Now, I practice standing and of course perform standing.