Having finally decided on track plan, mine is a terminus, and the layout of fiddle yard there comes the question of which end of the layout should one start building from?
Are the scenic section and fiddle yard on separate boards? Apart from wiring the fiddle yard is a breeze compared to the scenic bit. If they are on the same board I would loosely lay all the track first just to make sure your plan fits and that you don't have to make any compromises.
Assuming you have a scenic break (road overbridge, tunnel, buildings etc) into the fiddle yard, mark that clearly on your board and then go for the scenics, I would say.
I'd pretty much agree with what Nobby says.
I like to lay track first then start on the scenic areas. I divide the scenic up into areas and like to finish (or nearly finish) one area before starting on another.
Generally, I wouldn't want to be leaning over a completed area so probably start at the areas hardest to reach.