DS too young for this, but I'm interested to know what others do. Public toilets are rank because people treat them so badly. I usually wipe, then lay down tissue on the seat for myself - have been known to use antibac gel on the seat as well. I'm certainly not an anally retentive, obsessively clean sort (just come to my home and run your fingers over our shelves!).
Some people leave bodily fluids of all types on loo seats, I simply can't bear the idea of sitting on someone else's dried wee (or worse - boak).
So I think I would wipe for DC - if I had time, that is!
I always hold DD 4 yrs over toilet and don't let her touch anything. She always has her fingers in her mouth so hate to think what germs she could pick up from public toilets - she also washes her hands everytime.