I use ubuntu spotify on ubuntu 14.04 and there is no settings in windows behavior for minimazing system tray for ubuntu its a missing option since 1.0.1 -my Spotify version 18.104.22.1687.g9bd02c2d- and there is no solution for on spotify community or stackoverflow.
This situation bother me when i am working.I dont want to see spotify when i am browsing between windows.
I find a program on github named kdocker and kdocker solve my problem.
KDocker minimize every program to system tray and i change spotify desktop script for minimizing easily.
Create file named as spotify-minimized.desktop.
Then move to file to applications.
Start easily minimized spotify from unity dash.
You can change music directly from tray and kdocker hide spotify window.