I am contemplating on buying a silver wine decanter. I was told that if I were to leave the wine over a whole Shabbos, the wine would taste weird, and I was told empty it and rinse it after each use.
If I only intend to use it on Friday night, would it be hachona to rinse it out after the seudah, being that i wouldnt use it again until the next Shabbos?
As you mentioned in your question, one is not allowed to prepare (hachana) from one Shabbos for another Shabbos (1). However, one is allowed to wash dirty dishes if the mess disturbs them. Additionally, if the silver will get ruined, you are allowed to wash the wine decanter (2).
All the best
(1) Tehila LeDavid 302:6, Shemiras Sabbos KiHilchasa 28:77.
(2) Maharshag 1 O:C 61, Shemiras Sabbos KiHilchasa 12:2