I've only just found the resume render button from the drop down of render and slapped myself. Very useful.
I was then thinking how about a render onto last feature. Sometimes im happy with the quality overall, but get some noise in places (some reflective materials, shadows etc). How about a render on to last button, which allows you to change settings (i.e. chose viewport selection) then give certain objects a bit of a boost, without losing the overall render?