My 3 1/2 year old has only been out of dipes for about 10 weeks or so. When we first started going without, I put him in underwear and it seemed to confuse him, like he felt like they were similar enough to diapers that he should be able to pee in them. So I stopped putting underwear on him at all. Now we've gotten two indirect nudges from his preschool teacher -- once when he had an accident (only one so far!), she put a note on the bag of wet shorts that said I need to bring in replacement shorts and underwear, and then today he said that she helped him with his pants (first time I sent him in with zip fly and snap) and told him he should be wearing underwear.
What do you think? Is it inappropriate for me to send him with no unders? I asked him if he wants to wear them and he absolutely does not want to. OTOH, he loves it when I make him stuff and little homemade boxers are so cute, so there might be an upside to it. But if it were up to me, I probably wouldn't be pressuring him to wear them. I can't tell how serious the teacher is about it. FAIK, she doesn't really care.