I have been practicing SAT essay timing, and noticed that my spelling is really bad when I go back to review. How much can misspelling words lower my essay score? Should I try to use simple words that I know I can spell correctly?
Spelling only matters if it affects the grader's ability to understand your essay. Minor spelling errors will not lower your score if your essay is coherent and well-organized. I'd recommend asking someone else to read your practice essays to see if he or she is distracted or confused by your spelling errors. If not, you probably don't have anything to worry about.
Don't dumb down your language just because you're afraid of spelling something incorrectly. Knowing the meaning of the word and using it correctly is context is far more important than getting all the letters in exactly the right order. See this article for more details!
You should usually follow the spelling when writing, as this may affect the perception of your essay by others.