You can’t go too far wrong with a three column grid on a poster.
If it’s very wide poster, 5 or 7 or some other odd number of columns might be appropriate. But I will admit, a straight three column grid can all get just a little... rectangular.
Here’s a trick I learned from the Babylonians. The Babylonians are the ones we have to blame for our twelve hour days. The Babylonians like counting in twelves, rather than ten. Ten can be halved... or divided into five. Twelve can be divided into two, or three, or four, or six. Much more practical.
Create a grid that can be divided in lots of different ways. Or, to think of it another way, create a primary grid (three columns) and lay a secondary grid on top of that. Here, a two column grid superimposed on each main column. That is, six columns total.
Now you’re still following a grid, but you have more options for placement, and you can create a little more visual interest without sacrificing a disciplined layout.
You see this technique from time to time in journal articles, which might have a two column text layout, but will occasionally throw in a figure that is two-thirds of a page wide.
Better to make a complicated grid than abandoning a grid.