I think there are certain copywriting principles that every piece of copy should follow. I have written an article about eight fundamental copywriting practices. For a tone of voice document there are several specific grammar and punctuation conventions you can specify to get better consistency.
Language and punctuation
How do you want your brand to be referred to? Can your brand name be displayed in different ways? Is it an acronym, a bunch of letters that only you understand? Make sure you outline how your brand name should be displayed.
Some people have a thing about capital letters. A Product Name can be written with caps or you can just have the first word capitalised – Product name. It is a style choice. Same thing for titles and product names.
Do you prefer bullet points or numbers? Little thing like that are nice to keep consistent throughout your communications.
Salutations
How would you like to address your customer? It’s good manners to ask them but if you can’t decide on a form of address. Formal or informal?
Is it, Dear Mr. Girdlestone,
Dear Steve,
Hi Steve,
Should there be a comma or a full stop at the end?
Your choice on how to start and end your communications.