Mate we have palms galore where I live and do alot of palm trimming arfter our winter due to frost damage.
You need to cut the palm fronz that are yellow, burnt, scragerly etc of where they are attached to the main stem.
You should not trim just one part of a single fronz...........just lop the whole thing off where its attached to the main stem.
This may make the Sago palms look a little " bare", though i'm not sure on the extent of damage on your Sagos but I am familiar with Sago Palms (they are very popular in pool-scapes around here) and you will have to let the client that "you have to be crule to be kind" and that it's "exceptable horticultural practice".
This should also be followed by applying a quick release fert ( tropicals spec) You sell them on the fert app and let them know that the triming will "Promote new healty growth" and the fert app will help that "new healty growth" come on faster.
Hope this helps