I don't fly Boeing so am unfamiliar with the 737 anti-icing systems but on the Airbus we utilise the anti-ice system based on defined temperatures and not just observable moisture/cloud. The procedures we follow run along these lines......
ENG Anti-Ice should be turned on when OAT (on ground and for takeoff) or TAT (in-flight) drops below 10C and there is visible moisture (fog, mist, clouds, rain, contaminated runway, visibility one mile/1500m or less) in the atmosphere. The engine anti-ice must be on for all ground and flight Ops when icing conditions exist or are anticipated, except during climb and cruise when SAT is below -40C. ENG Anti-Ice should be turned on before and during a descent in which icing conditions are expected, even if SAT below -40C.