David F.
You should strain urine at anytime you think or have reason to believe stones are coming down from the kidney or more spacificly from the collection system. i've passed roughly 18 stones from the time i was 17, now 22. never havn't i had a batch to come up at a time. I never just pass one without knowing weeks in advance that it was coming down. when i say a batch i mean for myself 2-5 at a time. some people pass them in one shot, i'd like to know what thats like. if its taking many days/weeks to pass a stone be sure that you don't have a partial or full obstruction. this requires a surgical removal of the stone, not nessesarily a lithotriper intrument either. for me they went in me with a wire with a basket on the end and took the stone out. scaring is heavy, but almost inevitable with any procedure. but again, all your presumptions and questions are yes speaking as a patient. best of luck. if this becomes a chronic disorder or maybe less mild it might be associated with disorders known as Renal Tubulor Asidosis or Sjogren's syndrome to name a few. take care, best wishes.