I think yer supposed ta measure the "width" of the room and figure out how many full boards you can fit acrost it. You don't wanna have a tiny sliver of a board at the edge cause it looks bad and it might break/come free from the other boards when peeps step on it. The rule I saw was no less than 1/3 a board width.
I'd probs cut out the transition like Jeff293 shows from a full board butted against his line there (under the door itself), then any sliver bit'll be behind the sink and toilet and not be too noticeable.
If the flooring doesn't slide all the way under the door trim, you can cut off a hair of the bottom of the door trim. They make a tool for it, but you can use a hand saw for such a small cut; use a flat wood saw laid on a piece of scrap flooring and you should get a nice straight cut on the bottom that's the right height.
I don't think it'd look as "polished" if you were to scribe the floor to the door trim (rather than going under it) plus if it's a floating floor (not nailed down) then you need a 1/4" gap for expansion so it doesn't puff up at the board seams with house movement.