Yes, there may be a few lucky people who've been just fine doing it, but that doesn't mean that you'll have that dumb luck.
Recently on the Facebook group I was called a "pussy" for suggesting a girl should go to 7mm between 6mm and 8mm (and yes, it does exist) and was asked what was the worst that could happen by skipping sizes. So I'm going to show you

(ps. if you find this too 'graphic' then you shouldn't be stretching. If you're going to do it, you should be aware of the risks)
THIS lovely little set of images is the worst that can happen:
http://s260.photobucket.com/albums/ii9/ ... slideshow/
Happy stretching
