Do I need crushed rock under a rarely used rock paver walkway?
Do you need? No.  You could technically lay them down on grass and they'd probably be OK for a short period of time.  But the next person who rents the house will be annoyed, and so might the landlord when the pavers settle.

A 30kg of gravel is about $5 at my local HD - so how much money are you really saving here?  And lets say you save $50 - if they settle while you are still living there, it will cost you more than $50 to fix after-the-fact.

Do it right the first time.