Errno - needs resetting before checking or not?

From Coding Elements for Templates and CSElements - 11g Release 1 (11.1.1.8.0):

"The tags that are delivered with the WebCenter Sites modules and products clear errno before they execute so you do not need to set errno to 0 when you want to check for errors from these tags."

and then immediately after the example code block:

"If you want to check the results of the tags that are delivered by WebCenter Sites, you must include code that clears the value errno before the tag whose results you want to check."

So, which is it?