I have heard this admonition more than once (in fact, several times), so I added it to my repertoire of grammar rules (laws for us grammar goddesses).
The dictionary tells me that this innocent little word can be used as an adverb to mean:
However, ‘once’ can also be used as a conjunction to mean ‘as soon as’ or ‘when.’
Example: Once the vehicle is fueled, the driver is ready to go.
Yep. That’s right. It is not incorrect (double negative here, so watch out) to use ‘once’ as a conjunction.
I hate to be wrong, don’t you?
Keep in mind, however, that because we are thinking beings, we can still have preferences. My preference is to use ‘once’ to mean ‘one time,’ but I will not admonish anyone for using it as a conjunction.
There. I feel better now.