Whenever you are licensing a physical server by cores, make sure you only license the physical cores. Currently you are not required to license cores that show up as a result of hyper-threading being enabled.
Be careful how you check this as well. If you are looking at SQL Server properties or the sys.dm_os_sys_info table, it may show logical CPU which is not always correct. I am yet to find a script that accurately captures the physical cores but will gladly update this if I do. I usually check system information, look at the hardware specifications or speak to my hardware guys to determine the actual physical core count.
You definitely do not want to be paying for more licenses then you need.