Wow, I could open a can-o-worms with the house trap debate, but will not due to popular opinion on the matter...especially with septic systems. Codes in different regions have different requirements for different reasons. In Reading, PA you may encounter the suggestion to install a trap because it is common to the area. What you have to look at is the facts and details to determine if this is necessary.
Is this a new problem? Have there been any changes to the plumbing system lately? Has your septic tank been pumped within that last few years? Are there any sewage issues in the leech bed (where the waste water soaks into the soil after the tank.)? Do you have vents for each fixture or a main vent that goes through the roof? Does this happen during any particular event, such as doing laundry or running the dishwasher? Is there a vent pipe for the tank in your yard and can you see inside it? Is flowing or backed up water visible anywhere inside or outside the house? How many toilets do you have and does the smell seem to come from around a particular toilet (wax ring seal is compromised)?