Depends on the variety as to how you grow it.
No idea what Sara Ravens says about growing dahlias, but do you mean 5 shoots growing from one tuber?
If so, I would only let one shoot grow.
If you must grow 5 shoots then yes, you pinch out all of them, usually at the 5 leaf stage.

Thanks scrumpy. The plants in question are a couple of years old with clumps of tubers which I've split. I've deliberately reduced them to 5 shoots. Sara Raven has a web site & business selling dahlias, she also appears on GW now & then.

General rule is that the more "shoots" you grow, the weaker the bloom, but more of them. So if it's quantity rather than quality then what you do is fine.

Growing one shoot only with some decent varieties can mean upwards of 60 decent blooms per plant over a 3 month period.

Having a quick look at her site I would say she has three good ones

Downham Royal, Indian Summer, Hillcrest Royal. They are well worth growing.

But if you want good dahlias in a wider choice of colours you have to go to a specialist exhibition dahlia supplier.
Don't be put off by the word exhibition. You don't have to show them but the quality of bloom will be infinitely better. And they are just as easy to grow.

Have a look at Scrumpy@ blogs Thecat - when it comes to dahlias I'd back his advice above anybody's.

Thanks steragram. Judging by his replies I think your spot on.