For me, every time a *new* issue arises I go back to my lease and add language that specifically addresses the issue. That way, when it's time to renew, you're covered! For example, in a case like this, I would:
1) make sure all new tenants complete a "Rental Move-In Checklist" with a section that specifically addresses the condition of the bathroom/kitchen sink, toilets, etc; and
2) include the following clause in my lease:
"LANDLORD WILL NOT BE RESPONSIBLE FOR DRAIN BLOCKAGES AND BACKUPS IN THE KITCHEN SINK, GARBAGE DISPOSAL, DISHWASHER, WASHER MACHINE, TOILET, BATHROOM SINK AND BATHTUB CAUSED BY GREASE, PAPER, FOOD, HAIR, OR ANY OTHER FOREIGN OBJECTS. CLOGGED DRAINS AND BLOCKAGES ARE THE FINANCIAL RESPONSIBILITY OF THE TENANT."
Whether or not you want to enforce this, will depend on you. For example, if it was a first time occurrence and the tenants have always been responsible, I may simply send them a reminder notice letting them know that although their lease agreement specifically states that it's their responsibility, I will do a one-time courtesy unclogging at no charge. However, if you decide otherwise, you're covered by your lease agreement!