When should liquids be thickened to help with coice disorders?

Depends... In general, thickened liquids help patients with aspiration of thin liquids due to vocal cord paralysis, weakness, or impaired sensation in the larynx (voicebox). Usually this is from injuries to the vagus, recurrent or superior laryngeal nerves from surgery, strokes, tumors, infections, or other conditions that may affect these nerves.
Reflux. Children who have reflux or issues with swallowing like dysphagia cannot tolerate thin liquids. Thickening of liquids to honey or nectar consistency will make swallowing easier and aspiration less likely.