is this the letterhead for the individual submitting the letter? for example, if i ask my boss from company X to write a letter for me, would he use the letterhead for company X?
It probably isn't required for most schools but I remember that I saw a school somehwere that suggested having the writers put it on the actual letter as well. It wouldn't hurt but definitely not a necessity as @hoihaie pointed out!
My AMCAS ID won't exist yet while I'm getting most of my letter. I think interfolio allows you to send a cover page along with your letters though, so you can actually add it yourself (if you are using IF).