@WUSLS in my very limited testing (on Mavericks) shows that you need to set two keys: NewWindowBehavior and NewWindowTabsIdentifier. I had to create a bookmark folder that contained the tabs that I wanted, then used defaults write to set the Behavior and Identifier keys:
defaults write ~/Library/Preferences/com.apple.Safari.plist NewWindowBehavior 5
defaults write ~/Library/Preferences/com.apple.Safari.plist NewWindowTabsIdentifier 22BB2FA0-739D-486A-9572-F53DE49
Now obviously that Identifier string will be different for you, but once you have it that should set the tabs for startup. Getting the tabs you want bookmarked is a different story. I didn't do any testing on that yet.
HTH
Thanks for the information. Much appreciated! Sorry for the late response.