There probably an infinite number of answers to this one :) And it may vary by project as well. I am sure there are pros and cons for each approach.
I can say on my last project, "Turning The Hands Of Time" (www.imdb.com/title/tt3953804), I started with scoring the opening scene and then the end titles and at that point decided to go backwards through the cues in the spotting sheet until I scored the second cue. I generally do the opening scene first since it sets/defines the energy of the film (like an opening paragraph in a story or article) - I try to create a 'hook'. Then I often do the end titles to see if I want to define a 'theme' for the film which I can borrow against or play around in the cues. Though sometimes, l take the theme from a cue for the end titles and other times, I borrow music from one or more cues in the film and reiterate it in the end titles as a 'summary' for the film (sort of the reverse of an overture).
I decided to work backwards in my last film only because I never scored that way before and wanted to see how it impacted my writing style and the final product (not sure it did....).
Since my next film has not yet started shooting until this weekend and the following weekend and since I have the script, I am going to read the script (I have never tried this before), to see if it inspires me to any thematic style, allow me to jot down some ideas for cues and possibly write the end title music.