FHB touched on this at her day about ec. From memory she pretty much said that it is so prevalent that there's not really any point in routine testing - around 50% of the population would show positive results to the blood test, and there doesn't seem to be much/any link between test results and disease, disease seems to be much more linked to organ/tissue damage caused by the parasite and can appear a long time after the parasite has gone. She seemed to be much more in favour of doing a one-off course of panacur at the time of bonding to destroy any parasite that might rear up during that sort of time of stress. She even showed us a case study of a bunny that was ec -ve who developed kidney stones after a period of stasis/dehydration, but that this bunny had been ec +ve years before - so thought that the kidney stones were then able to form due to earlier ec damage, even though test results showed no sign at that point.
Personally I have tested for ec once for rehoming purposes, but that was because a potential owner particularly wanting an ec -ve bunny due to previous problems they had had. The test is around £60 so it wouldn't really be feasible to test every bunny that comes in - especially as it would only mean anything if you tested the bunny due to be bonded with that one too.