depending on how much water your changing I doubt it would have much of an impact. I have a septic system as well and dump about 70 gallons of salt water a month into it. Considering the septic tank is about 1500 gallons, and most of the water just runs off, the salt water is diluted by the rest of the water you use in the house. Also your bacteria survives the soap and bleach you use in the laundry, shower, etc.
That being said I use a product called Rid X, which is a bacterial booster for your septic field, it is in powder form, and you just dump the box into the toilet and flush. One box a month