Don't worry about it. A rejection of a paper and/or poster to a conference will not change your chance to get into grad school, regardless of who is on the committee. Well, unless it was just howlingly, embarassingly, horrifyingly bad or unethical in some manner (mass plagiarism of a published work can qualify). As an example, I was told a story of a time when a professor looked at a submission that was almost a full-scale copy-paste of that professor's own work. That is not the sort of thing a person is liable to ever forget, because it is just so brazenly ridiculous that you can safely assume that person would not ever agree to take on such a student.
Any serious high-level academic has had so many things rejected that it can beggar belief. A professor of note is almost certain to have had more of their work rejected than you would manage to submit during a whole bachelor-to-PhD process. The only career I've encountered where you have to deal with rejection more than being an academic is sales.
So really, don't worry about the prospect of rejection, it is the norm for everyone. Focus on whether or not your work is well written, sensible, and clear. Then send it out into the world and see what happens. If it is accepted, great; if it is rejected, great, at least you gave it a shot and hopefully learned from the process, and you certainly learn a little bit about what it is like to work in research, because it is not the last bit of work you'll get shot down.