In the present tense, don't need to and needn't are interchangeable in contexts like jacinta's essay, which refer to a specific situation happening in the near future:
"You don't need to bring anything to dinner - just yourselves!" "You needn't bring anything to dinner - just yourselves!"
When we're talking about general necessity, stick to don't need to:
"You don't need to know French in order to get by in Switzerland."
You can use the past forms needn't have and didn't need to interchangeably to talk about something someone did in the past which wasn't necessary:
"You didn't need to wash the dishes by hand. We've got a dishwasher!"
"You needn't have washed the dishes by hand. We've got a dishwasher!"
Both of these mean 'you did it, but it wasn't necessary'.
If you're talking about something which wasn't necessary and which was NOT done, stick to didn't need to:
"I found the definition in my dictionary, so I didn't need to post my question on the forum." (= I didn't post it).
Compare these two:
The weather was beautiful so I didn't need to take my umbrella. (= I didn't take it.)
The weather was beautiful I needn't have taken my umbrella. (= I did take it.)