On bisonoric systems, using only 1 symbol means marking or not marking every single note. Only repeated notes on the same keys and in the same bellows direction do not receive any marking (or deliberate non-marking). I feel that marking every note, whether using a line or a symbol, creates clutter.
If fingering is indicated, a circle around the finger number can indicate opening bellows. This is used in some Trikitixa (Basque diatonic accordion) schools. These symbols already exist for stringed instruments in music notation programs,
Using 2 symbols on bisonoric systems means marking bellows inversions. No notes after the inversion need be marked until the next inversion. This reduces clutter, although on an instrument or a playing style that uses a lot of push/pull playing technique, the staff will become rather busy.
On bisonoric systems, using 3 symbols again means paying attention to each and every note.
On a unisonoric instrument (bearing in mind that the OP was for EC), if it's somehow important to indicate "whichever direction you want", I'd agree with JimLucas's comment that,
Since most notes would be the third option, it would make sense for no marks to be the default "mark" for that, with two distinct explicit marks for the other two.
On the bandoneon, playing anything from 2 to 8 bars in the same direction of the bellows after an inversion is normal, so the staff remains uncluttered. On the Atzarin bandoneon, having every note repeated in the opposite direction of the bellows, means that whole phrases can be played either on the draw or on the push. I am currently assigning bellows direction based on the chord, as each kind of chord has either 1 shape both push and pull or 2 different shapes, where the most comfortable form, and consequently its bellows direction, is given priority.
However, there are some cases where it's really difficult to decide which is the more comfortable. It is tempting, in these cases, on this kind of instrument, to think that a symbol for "whichever direction you want" would be useful. In practice though, having this option is only good when preparing a piece for the first time. The learner must try a particular phrase both push and pull and make their own choice. After choosing though, I think it's better to mark the decision as either opening or closing. So one thing is indications for a learner and another is one's own indications as reminders as to how you play the piece.
Edited by Atzarin, 01 August 2016 - 03:40 AM.