On 737 Classic & NG, Boeing procedures always stated that due to any moisture at less than -40c being "likely" to be in the form of ice crystals, it was not required to activate engine anti-ice when in climb or cruise with SAT < -40c, but still advised during descent (assumedly because you are descending into "warmer" air more likely to contain liquid moisture)
Boeing has recently released a bulletin (covering all types) which details incidents involving icing/surges/flameouts caused by these "safe" crystals forming ice on impact with engines (guess pitots would be something similar) but stops short of recommending activating TAI in climb/cruise when SAT < -40c.
I find this just a little ineffectual, surely if they are advising of a real risk, they should be at least recommending you disregard the <-40 exemption ? ? or am I missing something here ?