You won't get the full effect from the weapon or gear if you're below optimal rank. It depends on what you were using before as to whether or not it's a bad idea. At 17THM, a rank11 decorated copper scepter was slightly better than upgrading to silver scepter for me. I guess at 18 or 19 the silver will be better. You'll need to change the items in & out and see what it does to your attacking stats.
Also, getting a higher rank weapon early does give you long use. Plenty of time to grow into it I guess. Though another drawback is if you like to use the NPC repairs, higher rank is often much more gil.