Drafts are for refining concerns such as organization, plot, characterization, etc., not for mere mechanics. What you tackle on each draft is up to you; personally, I tend to start with a general "let's see what I've got" pass to fix up obvious structural problems at the sentence, paragraph, and even scene/section level, while making notes about what I need to improve in other regards.
Ideally, your grammar and spelling would be just about perfect in every draft, because they've become automatic. Grammar and spelling are bare minimum skills for being a writer. You wouldn't try to be a carpenter if you couldn't reliably hit a nail; you'd practice hitting nails first. Similarly, if you can't always correctly spell the word you intend to use (typos† aside), you should learn to do that first. You will be a lot less frustrated by the process of revising, in my opinion, if you are not trying to deal with macro and micro issues at the same time.
My philosophy: assume that you'll run up against deadline and your current draft will end up having to be published, even if it's your first, and try to write so that you won't be too embarrassed if that happens. There are plenty of published writers who have inconsistent plots and cardboard characters, but very few who have consistently bad spelling.
† Typos are when a word gets scrambled (or lost) on its way from your brain to your fingers. Happens to everyone. Not knowing whether to use "their" or "there," or how to spell "minuscule" (an old nemesis of mine) is not a typo.