I would think reusing the needle would be ok, but I would not leave it attached to the pen. If you do the pen is open.
The needles are not expensive especially compared to all the other cost with using this drug; like the doctor visits, the labs, the Victoza itself. So why not just use a new needle.
Just the short amount of time I have been on Victoza I have noticed that if you leave the needle attached it will leak some, even if stored with the cap on and the needle pointing up.
Have your doctor write you a prescription for your needle tips. I used to work in a pharmacy and many times it would just be just be the cost of your name brand co-pay. When you reuse needs you increase your risk of infection (which as diabetics we are more susceptible to, not to mention if a needle breaks off inside you. Please be careful and no doctor worth his license should recommend reusing needles.