Ironing underwear is ridiculous and pointless. Unless you genuinely gain pleasure from ironing, which strikes me as weird, but each to their own....
Some things crease more than others. These things NEED ironing: cotton shirts, my nurse's uniform, bed linen, smart trousers.
Other things just don't: Jeans, Stretchy fabrics, most knitwear apart from very fine-knits.
Casual T-shirts and tops - sometimes need ironing....depends how quickly you get them hung up after washing, and how heavy the fabric is and what kind of style they are. Leave them in the washer after it's finished and they will definitely need ironing.
I rarely iron much other than my work uniforms. DH's work has a casual dress-code so wears jeans, Tshirts, etc for work, few of his clothes need ironing. Very few of my clothes need ironing. In the days when I used to do an office job in smart clothes I seemed to do a lot more ironing.
Hate it. Maybe if I liked watching TV I'd tolerate ironing more.