When a company gets bigger than a few people, there is no way one person can review everything that goes on. The bigger the company gets, the more you have to trust tasks to other people. The more people you depend on to get the job done, the more important trust becomes.
If you are the CEO of the company and something goes wrong, you will ultimately be held responsible. That's your job. There is no way you can check everything yourself, so your job is to create an organization that you can trust to do the job right.
Trust is Fleeting
If you are in a company of 50 people, your CEO probably knows you well, and can trust you personally. Depending on his/her personality, he/she may not care if you don't proofread your e-mail, because the CEO knows he/she can trust the content of your work (even though the presentation may have room for improvement).
For bigger companies, the CEO doesn't have this luxury. If he/she gets sent an e-mail from manager X of division Y, and manager X has 200 people below him/her who actually did the work, the CEO likely has no hope of objectively judging the quality work of manager X. So the CEO takes a shortcut and looks for reasons to be wary with the limited information he/she has.
Careless Mistakes are Endemic
That is one of the real reasons that many people put such a high value on good proofreading, spelling, grammar, etc. This is the reason that many people will throw out a CV if it has even one mistake on it. The ability to check a document for careless mistakes thoroughly says that the person is taking responsibility for something under their control. That's a good sign.
If Manager X in the above example can't even check an e-mail, how can a CEO trust that they can properly control the quality of the 200 people working below them? If the manager makes careless mistakes, what message does that send to subordinates? What does that say about that department's attitude toward checking work or quality control? It raises a bunch of red flags that say, "Caution!"
Does This Mean Good Spelling = Good Manager?
At the end of the day what matters to the CEO is that the job is done properly. And that is what is ultimately important to you and your company. If the CEO trusts that you have done the job properly with typos, then don't worry about it. If the CEO has ever pointed them our or you've received comments from upper management about that aspect, it's probably worth your time to proofread.
While the job being done properly is the most important, losing the trust of anyone who can give you a headache will ultimately force you to spend more time building trust, and less time actually doing the work well. That's counterproductive and should be avoided. An ounce of prevention is worth a pound of cure in this case.
For Safety's Sake
If an e-mail is around 250 words, you can probably read through it in a minute or two. Correct obvious mistakes, read it again, and within 5 minutes you'll likely have corrected 99.5% of all mistakes. That 5 minute investment always seems worth it to me.
As a bare minimum, I would always proofread the following:
- Proofread any e-mail that will get forwarded around (people who don't know you will read it and judge you based on the content, including spelling)
- Proofread anything that will go outside your company (customers will make judgment calls of their own, and good luck explaining why 5 minutes of proofreading wasn't worth it)
- Proofread anything that will go to your boss' boss or higher (you don't want to make your boss look like he condones carelessness)
- Proofread any numbers in any document ever (triple-check numbers, always, always always)
- Proofread any official reports or regulatory documents that will get filed away (again, judgments will be made on the little information that you provide)
Anything other than this is up to you.