Writers work very hard on their books and if they don't want to waste all that effort, it is essential that they present them in the best way possible. This means they must follow all available submission guidelines to the letter, and be courteous in all subsequent correspondence with the people they’ve submitted to: but there's more to consider than just that.
One of the first things that an editor or agent will do now, on the rare occasion that a thoughtful, well-written and appropriate submission ends up on their desk, is to run a quick internet search for the writer’s name.
This reveals a lot about the writer’s work: most publications and competition placements from the last decade or so should appear somewhere or other.
It also reveals a lot about the writer. Especially if they belong to writers’ message boards where the posts are open for all to see.
If their posts routinely contain careless errors or sloppy grammar, or are hectoring or bullying in nature, lacking in logic, or wildly misinformed, how do you think the agent or editor is going to respond?