It's really personal preference. If you have a spike that is turning yellow, none of the yellow part is going to produce any more flowers. You could cut it off just above a node that is still green, if there are any, but chances are the whole thing will go yellow and dry up soon. My own habit is usually that I will wait until the entire spike is dried up to the base, then cut it off. Even then, it's not necessary but more for aesthetic reasons.
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Cheri
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