Plenty of people choose not to walk at their graduation for one reason or another. Its not like you wouldn't earn a diploma.
Luckily, I went to a school that didn't have such strict graduation requirements.... Senior photos, however, had to be taken using their "male" and "female" fake tux and dress/pearls tops... essentially a one piece shirt that they draped over you to look like a tux or a dress. No way I was going to do that as a lady. No way my school was going to let me wear the male shirt. I said ->-bleeped-<- it, so now I'm that kid with no picture in the yearbook. I'd prefer that. Its kind of fitting of my personality anyhow, really.
Really at the end of the day not walking isn't that big of a deal... unless it is to you. I did walk in high school, but I haven't walked at either graduation since. I kind of wish I'd gone to the grad school one... but only because they gave out light up, color changing, maracas.