Is there any rule of thumb that guides runway choice when using an uncontrolled field in calm/clear conditions?
I know some publish default RWYs for noise abatement, but I'm curious if there's any guidance for when conditions are such that ANY runway may be used.
Is it 100% pilot's discretion? Is pattern direction pretty much established by whoever gets there first?
It really depends upon the airfield. At controlled aerodromes, the runway in use will be given to you. If it is vastly variable or nil (quite uncommon) then tower will probably select the runway that ensures the greatest flow of traffic on the ground and in the air.
At uncontrolled airfields, the wind sock dictates which runway to land on as well as any other traffic in the pattern announcing their intentions. If, again, the wind is nil, then you are free to choose which runway you'd like as long as you announce your intentions.
If there is no wind at an uncontrolled airfield, then other factors can influence your decision on which runway to use and rightly so...such as terrain hindering an approach or making it difficult, point at which you want to taxi clear of the runway, approach into the sun (not advisable if avoidable), runway slope (land on an up-slope), experience at the airfield and experience on a runway at that airfield, ground features to judge the approach profile (or lack of) and the list goes on.
None of that is written down officially as far as I know, but it comes under the general heading of "decision making" and "airmanship".
That said, if somebody is already in the pattern (at an uncontrolled field) and you then arrive, you must follow their lead unless you deem it unsafe to do so.
In the US (probably same everywhere else, not sure though):
Check the airport facility directory. They usually publish traffic patterns (left or right traffic) for airports/runways. If it doesn't specify, you are supposed to use left traffic; however, if there are no other aircraft in the pattern it doesn't really matter.
As far as runway goes, you can use whichever you want, as long it isn't closed. Usually try to fit in with the flow of other traffic.