I invest in older homes, short answer is yes, I think it's a good idea and I've added that to my list of due diligence.
Here's my long answer. What I've learned is that it's not always a black and white result at the end of the day...inspection good/bad, pass/fail. You're not going to walk away exclusively with feelings of pain or pleasure as-in feeling like you "paid for nothing" or "overpaid" if no defects in the sewer line are found OR feelings of "it was worth every penny" or "thank god I had it done" if defects were found. The intangible benefits you get is an education on the sewer line for that property specifically and of that vintage generally. You get answers to many questions. Does it still have a whole house trap? Should I get it replaced in the future? Do you as a newer investor know what that is? What kinds of plumbing materials were used in 1900? Are you able to get a video scope down a sewer line with a whole house trap? Getting the inspection also affords you access to the inspector/plumber who is looking at hundreds of sewer lines everyday, probably of that same building age. Take that opportunity to ask them if this inspection is typical or our of the ordinary with what they see, and why?
Update the thread with what you decide!
Good luck!