I have mine on the loosest hook its most comfy for me.
Most Helpful Girl
It should always be on the loosest hook when it's new. As it stretches out, you gradually use the closer hooks so that it still works as a bra. If someone is hooking their bra on the tightest hook when it's brand new, then the need to go down a band size and up a cup size. (So if you're wearing a 34C on the tightest hook, you should try a 32D on the loosest hook instead.) Going down a band size and up a cup size (or up a band size and down a cup size) is what's called sister sizes. So I'm a 34DD but I could go to either a 32DDD if I shrunk of a 36D if I gained weight.
So, yeah, some of my bras that are newer are on the loosest hook while older ones are on the middle hook.1