If tree roots have managed to find their way into the sewer pipe and expanded their entrance hole through sheer growth then the pipe may be compromised enough that any poison poured into the sewer may leak out and over time make the soil around the pipe rather toxic to any tree root growth there. Also fine soil particles will be able to enter and accumulate unless there is a constant flow of water.
Presumably the plumber found lots of roots because the penetration had been going on for some time. If he were to repeat the process in say two years time then he would find a lot less root growth. So an alternate approach which does not poison the soil might be to investigate a way of accessing the underground pipe from a convenient location to make the plumber's job really quick and straightforward. If you work with a reputable plumber on the job then he might be willing to give you a special deal on regular root removal.
There is also a technology that can insert a sleeve inside the existing sewer pipe; tree roots might still grow into the old sewer but be unable to penetrate the inner sleeve.