Everyone else I've spoken to pulls them out in March/April because
they never grow back. Anyone have any more info??

The mum that I planted in front yard has come back fine. I also see
that mum in my neighbors' yards also come back for the years that I
have lived in this are (northern New Jersey).

... and the guy at the store told me to trim them in the spring
through till early summer, and they should come back.

I believe the idea is to prevent it to become too leggy. Otherwise,
the top half loaded with flowers with become too heavy and will fall
to the floor. I start trimming mine this year, and it surely looks
bushy than what it was last year.

Hope yours will do OK.

Jay Chan