I don't cut them back after frosts and haven't heard of that advice before. but I certainly cut out spent flower stalks and as the plants don't form seeds they throw up a second lot of smaller flower spikes later in the year.

I deadhead mine regularly, as said they will produce more flowers albeit a lot smaller, once they stop I cut them right down to the ground and they always give me a second showing, last year they flowered three times but that won't be happening this year as they are lasting so well there just won't be time..