Dazmundo
Can't talk for the 737 but what follows is posted with the benefit of having flown other Boeing types..
As far as the anti ice system itself is concerned if it's working OK automatically (the detectors and the plumbing) we leave it alone to do it's own thing....there's no FMC entry required or switching required. If you are using manual anti-ice ( on an older Boeing or on a modern one if the ice detection system is inop) you turn the thing on manually when you see you are encountering icing conditions. It's pretty much what OhNoCB said..for descents it's look out the window, use mark 1 eyeball, and monitor the temperatures ( which Dave alluded to).
As far as the programming of the FMC itself is concerned TBH we rarely bother with entering an "icing level" in the FMC ( I accept others may do otherwise) - on the Boeings I have flown all that FMC entry does is modify the calculation of top of decent point and the descent profile (makes it shallower) to cater for the higher idle RPM you get with the engine Anti Ice on, i.e. the FMC entry only effects number crunching in the FMC, it doesn't have any effect on the operation of the actual icing system.... ...... if you do then end up with engine anti ice on in the descent and a higher idle RPM, and hence a shallower than ideal descent you can always use speedbrake (
) to get back on profile.
In short the FMC entry isn't "mission critical".