Don't Wait for a Mainline Blockage
If you have a clogged sewer line every year or two, don't just keep getting it cleaned. Most sewer service companies will 'scope' the line to see what's causing the problem. That entails sending a remote camera down the line for a visual inspection.
The recurring issue could be tree roots entering through clay tile joints, a partially collapsed pipe or even rotted-out cast iron that allows soil to fall into the line and create a blockage. It's worth paying to find out the cause and get it fixed rather than hassling with frequent sewer backups and sewer-cleaning bills.