The glider pilots at my club all agree that nearly everyone has a "natural" turn direction, and we split about 80% left, 20% right (no apparent connection with right- or left-handedness). Almost all gliders are flown from the centre line, so that's not the reason.
I became ambi-circular by practice (this is worthwhile in gliders as if you share a thermal you need to circle in the same direction as the other gliders for obvious reasons!).
However, some days I start a flight circlng better one way than the other, except nowadays it's not consistent which way. After half an hour or so, it seems to even out.
Around 1/3 of a glider flight is usually spent circling, so we get a fair amount of practice at it, in my case usually between 30 and 45 degrees angle of bank. In a weak thermal, any inaccuracy can put you on the ground, so accurate flying is at a premium - thus the awareness of this phenomenon and the practice to fix it.
I've very little time in power, but in my few flights (some from side-by-side aircraft) never had a problem circlng in either direction, though always by visual reference and feel - to me, the instruments are too coarse for accurate flying unless you have to rely on them in IMC.