No, there are no real drawbacks to not fully utilising the disk. You are not changing the geometry. Partitions may fill earlier, but you should not be concerned about wear levelling as the controller should handle that in the background.
So, only that you will have less overhead space. Are you using a swap partition ? You should put this at the end of the drive, it will be one less to move during a resize. Sometimes it is just easier to pick a number. If you just make it EXT2, you can store backup images of the partitions in files inside ?