Some people choose not to connect the vent tube to their AGM batteries cause they think they are fully sealed. In reality, there's still vent holes in them, and even the paper instructions that came with my AGM replacement stated to connect the vent tube if your car has one. The red cover on the battery also had a plug to place on the opposite end. Pic of my installation below. Will you run into any issues if you don't connect it? Probably not, but I'd rather not have a venting problem in my trunk, and it wasn't hard for me to connect it...
