Depends on the bike. You can leave some with the key in the ignition and no one would touch them, while others will get snatched from a bank vault guarded by a SWAT team. Pretty much any 500 older than a few years would fall in the first category, any new SS in the second.
- Alarms are all but useless, good only to deter kids from sitting on the bike.
- Chains, locks and other immobilization gizmos are a mild to moderate deterrent. They just slow determined thieves down (not by much) while they are a pain to carry/use every day.
- Security by obscurity, or "out of sight - out of mind" approach can be somewhat effective, but it depends on a lot of factors.
As already mentioned, if you insist on buying a brand new thief magnet have it properly insured. Get an alarm/lock just so you can say you didn't make it easy for the thieves. I would use Xena/chain combination. You can leave the chain in the garage and take Xena with you for parking outside.