I seem to recall this very subject be discussed a few years ago with regard to some high altitude engine flameout events involving The Beechjet 400. (now known as the Hawker 400XP) In those cases, ice crystals encountered in (seemingly) clear air have been identified as the cause of the flameouts. AFM procedural revisions were issued to address the problem. Activation of engine anti-ice when in the vicinity of convective activity (even if in VMC) is now required.
Several large airliners equipped with certain models of GE CF-6 series engines have suffered flameouts as the result of ice crystal accretion as well.
The conditions requiring engine anti ice use vary by specific airplane and engine type, so a universal policy is not practicable. The proper procedures and conditions for engine anti-ice operation will be contained in the AFM. In some airplanes where maximum cruise thrust may be limited by ITT or N2, engine thrust reduction may be required prior to engine anti-ice activation. In some circumstances of WAT, this may necessitate a descent to an altitude where the reduced thrust will sustain a safe minimum airspeed.
I found this 2008
Flight Safety Foundation article on the subject interesting.
westhawk