I think this depends somewhat on where the student is at in their education. I'm currently an undergrad, and no one seemed to expect me to have poster or publication experience. I did submit an abstract to a conference right in the middle of applying, so I was able to put a submitted entry on my CV for my last 4 applications, but I don't think it really made a difference. However, I do have plenty of research experience and have my own projects, and my 3 letters were all from research advisors who know that I can be an independent researcher. I'm pretty sure that was the most important aspect of my application.