The problem arises when you're not sure if it's read as letters or as a true acronym. An example is HID (human interface device, e.g., keyboard, mouse, or track ball). Is it read as 'hid' or aitch eye dee? When I run into one of these when copy-editing an engineering report I'll try to find and example where it doesn't have the indefinite article and ask an engineer to read it to me.