This is a question about interpretation and phrasing, which is always a subjective and personal field. However, there are objective rules of thumb that govern phrasing that are relevant here.
- We're always told "the downbeat is the strong beat." This is especially true in the baroque period, often referred to in string contexts as the "Rule of the down-bow," which dictated that the "strong" downbeat should use the "strong" down-bow direction. Like all rules, it's made to be subverted and shouldn't inform every single measure equally, but it was universally understood. Charpentier (quoted in Cessac, quoted in Judy Tarling's Baroque String Playing for Ingenious Learners) says in 1692:
Note that there are strong and weak beats in music. In a measure with four beats, the first and third beats are strong, the second and fourth are weak.
By this rule, notes "0" and "9" would emphasized, but "0" more so than "9" as the downbeat.
- We might quibble about the word "accent." The ">" accents printed in the Bach excerpt are purely editorial; they were not put there by Bach. (See the manuscript, https://ks4.imslp.info/files/imglnks/usimg/d/dd/IMSLP457551-PMLP05948-Partitur_D-B_Mus._ms._Bach_P_415.pdf, page 7.) (As a side note—I'm certainly used to hearing those very heavy-handed accents in that Prelude. I'd be curious when they show up in the historical record, and in their place in a reception history of the piece.) Especially as a string player, a modern-era definition of "accent" has implications for attack and articulation that are not germane to a discussion of metric emphasis and phrasing. I'd prefer to use the term "emphasis," or the historical terms "strong" and "weak." Also, talking about accents suggests isolating individual 16th notes for "special treatment," implying that all the notes are at one volume and then certain ones should "stick out" as being much louder. This assumption of a heterogeneous dynamic level is not part of baroque thought.. To quote Tarling again:
One of the main concepts associated with performing Baroque music, in technique and inherent in the music, is the principle of inequality. ... By emphasising different parts of the beat, the bar and the phrase, this way of playing has no need of detailed dynamic instructions.
... in other words, Bach had no need to notate any accents because they would be understood from convention.
Instead of thinking of "accents" that affect specific notes, I would advise thinking of phrasing fluidly; if a note is "important," chances are you will
build toward it and fall away from it. Instead of 2 "accented" notes
among 14 equally un-accented ones, I'd look for a dynamic phrasing
that took into account motion toward middle beats, toward downbeats,
and then across measures as the harmonic tension ebbs and flows. That brings me to
- Harmonic motion is even more important than metric stress to phrasing. The chord shifts with every bar, and you should note just emphasize certain notes, but sculpt your progress toward moments of increased harmonic tension (dissonances, diminished chords or chords containing 7ths, suspensions and other non-chordal tones, etc.), and relax on their resolutions. In other words, while the downbeat of every measure is (probably) the strongest, not every downbeat is equal. Your question focuses on the right hand, but sometimes this must be driven by the progression of the left hand.
