It’s good to catch as many typos as possible. It’s not always easy to see them, even with spellcheck running. And spellcheck can’t always see when you put words in the wrong order. Proofreading can be hard. Enlist some allies as early as possible to help read through and spot what you might look past.
If you’re reading through your thesis to prepare for your viva, don’t worry about miskates too much. If you find erors, you can always correkt them l8r.
Instead, focus on anything that seems vague. If you read something that you’ve written and think, “Hmm… What did I mean here?” then spend time unpacking what it’s all about. Typos aren’t vague: when you see a typo, you know what needs to be there. Passages that are vague are harder to crack: it may not be immediately obvious what needs to be there. But if you focus on the vague then you’ll be in a better position in the viva to talk about your research.