err on the side of caution or take a guess. And rivers

This is a fairly controversial topic.

My 2 cents:

• in the days before we had high resolution aerial imagery it was common to use highway=road for ways that hadn't been surveyed in person, however that classification is not really useful over a marker that something is there. Today, if the imagery is of good quality I would use a reasonable classification that errs to "lower" values (aka I would not tag a road through a residential area as primary just from imagery).
• as to extra work: one of the most annoying things you will come across on in OSM is arm chair mapping (aka tracing from imagery) that is not appropriate to the quality of the imagery avaiable. Vast expansives of houses with details that are simply wrong and offset dozen of meters, because they where traced from non-aligned and blurry imagery. So if you have aligned your imagery and it is of good quality please feel free to trace details from it, if it isn't and there is no overriding reasons to do so (HOT activiation or similar) please don't, down the road better imagery will be available.
• river banks: it really depends in the size of the river, as a tendency I would simply tag with width if it is essentially a straightline and under 20-30 meters. But as I said above, often it is more useful to add an object roughly and generalised than to prematurely add detail.