I usually wear a sports bra under my leo for practice because some of my practice leos are thinner than others, you know what i mean? For competition I don't wear anything under my leo just in case I get a deduction or something. Although I see other teams wear bras and underwear all the time.
Theres a girl on my team that has about literally 100 leotards! Im not lying the reason she has so many is cuz she buys one almost every week and cuz her aunt buys them for her at competitions but im not jelous cuz i think shes spoiled!