Venting into sewer line...

A basic understanding of how your septic system is plumbed will probably help you make your own decision here.

Take a straw, put it in a glass of water, put your finger over the hole in the straw and remove it from the glass of water. The water stays in the straw. Take your finger of the end of the straw and the water runs out.

Your, and every other, septic/sewer plumbing operates on the same principle. There must be places for free air to enter the drain pipes or the water will "lock" in the pipes. This source of free air comes from the vent pipes that go thru your roof. Since we don't want toxic/smelly air entering our living space, each drain pipe has a "trap" near the drain. This is the "S" shaped section of pipe under your sinks etc. It holds water and prevents bad air from coming back up the drain pipes. Bottom line, none of the air you blow into the drain lines is going to exit into your house, just the same as the smelly "sewer air" doesn't enter your house.

Forcing air into the sewer pipes means the air will follow the path of least resistance until it eventually exits somewhere. If you have a septic system, a small amount of the air will go into your septic tank and drain field, but most of it will simply vent out the pipes in the roof. Essentially, this will be the external source of your odor, so you need to determine what will happen with the smelly air once it exits the roof vents. You need to look around at what is downwind from your home and if the odor would be detectable before it dissipates.

Venting into the sewer line would require either running your vent fan 24/7 or capping the line when the fan is not on. Sewer gas contains some pretty nasty stuff that you really don't want in your grow room, or anywhere else where you are breathing the air.