I read two very similar blog posts last week: this one from Janet Reid, and this one from The Rejectionist.
Both make the point that it's important for writers to take care over their writing: to use the words they mean to and not ones which sound the same; and to check their work carefully before submitting.
I'd add that writers should extend these habits to include all written work they make public. And yes, that includes emails, tweets, and comments on message-boards and blogs, because those online comments are tied to our names. They stay around for a very long time and just supposing that an editor or an agent types our names in to Google, do we really want them to discover that we're often sloppy and careless with our words?