[atlas] Anchors and NTP back to the future

Some anchors appear to be reachable with NTP but most of them are not. I guess it means the anchor always has a NTP server (and I assume it is on purpose) but many networks filter it. What is the good recommanded practice? Specially in the light of the Back to the Future attack <http://www.cs.bu.edu/~goldbe/NTPattack.html>