I'm working on a direct mail list using unpaid property taxes and It occurs to me that in the area in which I am searching they don't assess a late fee until the calendar year ends. For those who have experience with this, should I just wait for say mid January to put effort into this sort of list? I feel like a late tax bill holds more weight once it's truly "late".

You need to determine exactly when the county considers the homeowner delinquent and how the process works from there. Then you will need to determine a criteria to filter the list depending on your market stats and demographics. 

I have a tax delinquent list that I filtered for those properties that have been delinquent for 2+ years, and owe 5k or more. My reasoning for this criteria is that New Orleans has a median household income of just $37,000. If someone owes 5k or more and has been delinquent for 2 or more years, I think they will be unlikely to come up with the money. 5k is about 13.5% of the median household income. That is a pretty substantial portion of their income they would need to save in order to be able to pay that off, so these people would be more motivated to sell.