The answer depends on how much you experience "important mail in spam folder" at all.
And regardless, you *can* set the spam folder "to sync" (so it's checked every time the app checks mail, same as the Inbox is for example) and is also included in the smart folder:
- Long press on the account on the app's main screen -> options and folders -> folders
- Wait a little so it refreshes the folder list
- Find the Spam folder
- Tap its name, choose "Sync as Spam"
- Tap the "settings" icon on the right, enable "Show in smart folder".