Yes. Once it’s installed, you do not need to launch SpamSieve yourself:
In order to avoid SpamSieve showing the Settings window when it’s not wanted, we recommend that you do not add SpamSieve to System Settings ‣ General ‣ Login Items ‣ Open at Login. It should only be under Allow in the Background.
If SpamSieve is not auto-launching as it should, please:
Although the SpamSieve application must be running in order to filter your mail, it ordinarily runs in the background with no user interface visible. The Dock icon can be hidden if desired, and you can also set it to quit when your mail client quits.