I've only just started Iaido in January, but will offer what knowledge I have gained so far. The style we study at our Dojo is Muso Jikiden Eishin Ryu. We are in Seiza a fair bit and fortunately permitted to wear knee pads (cheating I know). The first five forms in our school are Mae, Migi, Hidari, Ushiro & Yae Gaki which traditionally all start from Seiza. We do perform these from standing sometimes, but it is not the norm. For what it is worth, I struggle more with my instep and toes rather than the knees as my feet have high arches and clawed toe (stretching helps). It's an amazing art, beautiful to watch and well worth try it if your able to. I would discuss these issues with your would be Sensei before starting. Good luck with your endeavors.