Hi there, I suspect you should use it sitting down just in case. With vision occluded and your visual system telling your brain you are moving, that could upset your balance.
If your vestibular system is under-sensitive I would expect you to have less problems with motion sickness than most, since you are not actually moving. However if you are over-sensitive and prone to motion sickness, the mixed signal VR gives you cause you to feel nauseated. I get this effect mildly when I use it, and prefer slow paced games/environments.
If you have any concerns about seizures, don't use it.