Having a main sewer line periodically cleaned out is a good idea to keep the sewer line clear and prevent a backup.
I have my sewer line cleaned every 3 years which is good enough for me and when I've had it cleaned out they cleaned out some roots etc that could've eventually caused the sewer line to clog and backup.
It only costs me around $60.00 to have the sewer line cleared and cleaned once per year and it is well worth it to prevent a sewer backup.