The best way to avoid grammar and spelling errors is to use the following rule: Read your SIM out loud from start to finish at least once. If you can't read it out loud, then read it from start to finish, in your head, three times, slowly.
AND, if you have a spell-checker, USE IT!! It will, at first, catch a lot of words that aren't normal like "Bajoran". Just click the "ADD" button, and it will add those words that you know are correct to the dictionary. So, if it gets the word "StarBase" and you KNOW that's correct, then push ADD, and next time, it won't point that out as a problem.
Good spelling and grammar are imperative to making your SIMs readable. When one spelling problem becomes twelve in your SIM, it makes it hard to follow what you're trying to say, thus making the whole experience of reading the SIMs not enjoyable. So PLEASE PLEASE read your SIMs back to catch these problems!!
Here are some common errors that people make, and how to fix them: