Current plumbing code requires that each individual fixture is trapped, therefore you don't need a house trap. Your septic tank usually ties into the venting system of the house and helps prevent the buildup of gases within the tank that can make it go 'boom'!
If you're getting an odor in your house, check for floor drains that have no water in them. If you don't use floor drains, they can dry out and let sewer gas into the house. Also check other fixtures that are not regularly used.
Tips on odor from floor drains