There are no rules in filmmaking. Only sins. And the cardinal sin is dullness.
Despite being a bit of a movie nerd (I paid my way through university working mostly as a movie theatre projectionist), I’d never heard this quote before, and love it. And it applies so well to conference posters as well as films.
About the only rule about poster making is that your poster has to fit within the allotted space. Beyond that... you have free reign.
Yes, conference organizers may tell you that you need to have an abstract. An institution may grumble about whether you have the institution’s logo on the paper. But I have yet to hear anyone say that they were stopped from presenting a poster because of such thing. I often joke, “There are no poster police!” or “Let anarchy reign!”
But... as Capra knew, “no rules” does not mean everything is equally good. Some ideas are better than others, or work better in some contexts than others. And like movies, a dull conference poster is – in either the content or the design – is a sin. The good news is that dullness is only a venial sin. You can do your penance by making your next poster better.