Here is where I think the problem is. Maybe you should not have your tenant responsible for plumbing or any repair issues. However in your lease you should have a clause that states that any problems due to tenants negligence is then the tenants responsibility.
We just had a drain inspection done which showed from every point of water, i.e. toilets, sinks washer, that when flushed ect. that the water went whooshing through the drain. Of course if you have a company do this for free they will try and sell you something, obviously. However you will know that you are handing the property to the tenant in working condition and the last tenant did not leave any problems behind.
So, when the tenant calls for a stopped up toilet you know that likely something went down that should not have. Also you are on the property with the sewer service person confirming why, what and how.