When repotting, you can also cut off the old pseudobulbs that have lost their leaves. Oncidium bloom spikes should not be cut off until they are dry. They will rest, then spike out and re-bloom. Dendrobiums are a funny orchid. After blooming, cut off the flower spike.
Dens tend to drop all their leaves and leave nothing but the cane.
Do not cut the canes as long as they are firm and are not dead. This is where your next flower spikes will come from. You should check each genera you are growing for their individual needs. Any good sulfur powder is good. Of course this is a personal preference.