Scotchguard is a wasted product on carpet and rugs. It will wear off unevenly and when re-staining happens, it will actually attract dirt and cause the rug to appear dirtier than if you did nothing. In addition, as the Scotchguard wears off the appearance of the carpet will be streaky. It is a product with lots of hype and little to back it up. I worked for two high end carpet manufacturers calling on designers and architects for many years and I do know what I am talking about (at least on this issue!! :-))
Topically based carpet finishes do not work, The only finishes that work are those put on the carpeting during the final finishing processes at the plant. These finishes are standard on many premium commercial and residential broadloom and rug products.
Save your money and clean the rug as needed with your steam cleaner.

Scotchguard is good only if you have a big spill and can get right to it and clean it up. What you will find is that the real problem with a carpet is just daily walking on it. Scotchguard does nothing for soiling.

Don't bother. When I purchased and had carpeting installed, they offered Scotchguard service as p[art of the installation, at an extra fee of course. I went for it and found it did nothing to prevent stains. My kids spilled pop on the rig, my animals did their things (several puppies went thru my house, plus an elderly cat with vomiting problems). I even spilled a cup of coffee myself. Stains formed almost instantly. I consider Scotchguard a waste of money, a scam.