Both forms are fine - and interchangeable in most contexts. To my ear, the 'don't need to' suggests more urgency or personal interest:
'Hang on - we don't need to type in all the addresses': the speaker really doesn't want to do it, because there's a better way. But the manual might say this:
If you keep the addresses in a database, you need not type them all in manually every time you send a circular. You can simply use the Mail/Merge facility.
The rule seems to be that after inversion (need not or needn't) you don't use 'to'. But rules aren't really my thing, so I'll defer to other teachers.
Thank you, I'll stay tuned to the advice of the sage if they would rally round. But this is reasuring me of a fact I wasn't explicitely conscious about. I had been using both the above forms for a long time until I became aware of the apparent inconsistency.