I've been riding for three seasons and probably have 10,000-ish miles under my belt and I STILL CAN'T STAND UP! My friends think I'm an idiot. However, I ride very hilly terrain with some hills around 15-20% grades, and I don't have any problems getting to the top. So IMHO, it's not absolutely necessary to be able to stand up. It makes me feel better to imagine that I have extra super strong legs because I can climb hills withouth standing. I have no idea why I can't do it...it just feels un-natural to me and I have poor balance, although I could easily do it as a kid. At this point I've pretty much given up on it.