At our daycare the teachers are expected to help if the child needs them to help, is the short answer. Doesn't matter how the child is. In saying that, the Toddler room takes children from 2 to 3.5 and the teachers help them, but I'd imagine that there is less help offered in the Preschool room where the children are 3.5 to 5.
There is one particular teacher that seems to have an issue with wiping bums at DD's daycare though. It was the same back when DD was in nappies, this teacher would frequently leave poo on her bottom after changing, and she is just the same now that DD is toilet-trained! I even ask DD sometimes if this teacher helped her go to the toilet and DD always says yes, so I know it is the same teacher all the time. One day last week, DD had an accident and did a poo in her knickers. I got the knickers out of her bag at home and found a huge, well-formed chunk of poo in them - I mean seriously, how hard would it have been to chuck it down the toilet before putting the knickers in the bag?