I am also going through "in progress' documents and I wish people would submit most of them for review.
Personally, if the page is not completely transcribed it should be left as in progress. If the page was fully transcribed with many [?] it is up to the transcriber. However, I have seen perfectly transcribed pages with one or two [?]. Those should be submitted for review in my opinion.
In the long run, it doesn't matter. I will review all of the in progress and then go to those needing review. My strategy is to start at the beginning of a section and work my way through at that point. That way I can also scan completed pages. I am not sure how "perfect" LOC expects these but my feeling is that they should be as close as possible. Otherwise, the pages are not useful. I sense that LOC wants perfect but doesn't want to put undo pressure on citizen transcribers.
Transcribing as much as possible, as accurately as possible is always the goal. If you come to the end of your ability on a page feel free to save that page. That page will still be available to other transcribers to continue that page.
I agree with Henry, submitting a page for review with a few [?] unknown or illegible notes is a good idea!