The accepted answer states proposes the middle finger on the key bass to start. It is listed under "as a general rule (for my songs so far)". I tend to suspect that the parenthetical remark points to little actual experience underlying the urge to provide a helpful answer to another beginner.
That's actually a rather strange starting position: the typical Oom-pah accompaniment using (lower case chord, upper case bass note) C-c-G-c (4/4) or C-c-c-G-c-c (3/4) can be fingered 4-3-2-3 and 4-3-3-2-3-3 respectively (assuming that thumb-index-middle-ring-pinky are numbered 1-2-3-4-5). While minor key Oom-pah C-cm-cm-G-cm-cm can be played well using the 4-2-3-2 pattern, it is more usual to revert to 4-3-4-3 here: minor keys tend to involve a lot of switching between major, minor, and seventh chords, and playing the basic minor pattern starting with 4-2 rather than the major-typical 4-3 requires planning ahead for comparatively little gain. One could make do by playing everything 4-2-3-2, but that makes for crowded fingers in the major chord scenario.
Apropos seventh chords: you don't want to be playing the bass note with the middle finger (second row) when the actual chord to be played with the index finger is in the fifth or even sixth row.
Don't take my word for it: just look at a number of videos from convincing players: while they are rather flexible in what kind of fingers they use while playing runs etc, the basic action starter is almost always the ring finger.
Note that this is for playing the customary 2+4 (2 bass rows, 4 chord rows) layout. If you are playing 3+3 (some French button accordions, and quite fewer Italian ones) with a focus on bass runs, the starting position might conceivably be different. And don't get me started on the Belgian bass system...