There are two words in the early Zodiac letters that I've always believed strongly support the idea that the majority of Z's spelling mistakes were unintentional. In both cases, Z attempts to correct spelling mistakes, which would seem to contradict the idea of him generating purposeful mistakes.
1. The word CIPHER is botched the first time Z attempts it in his 7/31/69 letters. In what I strongly believe is the first letter written of the 3 to the Vallejo Times-Herald, Z spells the word as cyipher as if he is unsure whether to use a y or an i. To me, this is an indication that he started to spell the word with a y, realized that it did not look right, then added in an i because he was genuinely unsure which phonetically equivalent letter was used for the word. He then nails the word later in the same letter, the other two letters on the same day and every future attempt.
http://www.zodiackiller.com/VTHLetter1.html
2. The word ADDRESS in the next letter in which Z signs off as "NO ADDRESS". Here he clearly wrote the word as ADRESS and squeezed in an extra D once he realized that it didn't look right. The mistake is entirely consistent with his frequent confusion about when to use double letter combinations.
http://www.zodiackiller.com/ZLetter3.html
Another example is the attempts at the word BUTTONS. The first time it is used, Z wavers between spelling it with one and two t's, seemingly unsure of whether to use the double t. In future letters, the word is spelled correctly several times, which I would suggest is the result of him deciding from the first letter that "BUTTONS" look more correct.
I know I'm in the minority with this belief and I do certainly think it's possible that some words like "cid" and "doo" were purposely misspelled but I think the consistency of Z's phonetic spelling mistakes and his apparent attempts to correct his own spelling indicate that the majority of spelling errors were exactly that... errors related to his uncertainty about some of the phonetically equivalent options that make spelling difficult in the English language.

