naive metalworking question

If I'm going to beat a flat sheet of metal into a dished shape, is there any particular reason to use a flat hammer and a curved dolly or form, as opposed to using a rounded hammer and a flat surface?
I have always used a high-crown hammer and a flat beating surface (anvil, plank of hard wood, etc.) to raise the center in a flat panel. The edges of the panel curve up toward me as I work the metal into shape.
But I've noticed that when I see guys beating metal on TV they seem to use methods that involved a high-crown dolly or form and a low-crown hammer. Planishing hammers and english wheels work this way. The edges curve down away from them as they work the metal.
Is there a particular reason to use one method vs. the other?

 

A lot of guys on here know more about metal working than me and I'm sure they will chime in.

The main difference that I see is that it's a easier to hammer it down over a form. You don't have the edges curving up and getting in the way of your hammer. If you hammer it over a form the edges curve down out of the way.

Not a big deal unless the piece is pretty deep. I'm sure some of the other guys can offer more reasons.

 

I,ve done it both ways - I have my helve hammer setup so it curls the piece up as I shape it and my planishing hammer curls down as it works so at least to me it makes no difference . I'm sure there are some purist out there that will set me straight though .
One trick I use to crown small pieces is to pound it over a thick sheet of lead works great, better than a bag for small stuff .
I'm a Joplinite too so let me know if you need any help I've beeen bending tin for years . PM me for contact number

 

We had a hailstorm here this past spring. It left many cars looking like someone went after them w/a ballpeen hammer. My guess is that using a curved dolly surface allows adjacent "hits" to overlap easily.
It is always a skilled hit w/a curved hammer, that does not leave an edge mark. The edge mark would be a sharp change in workhardening if it was even accidentally carried out. Perhaps the curved dolly makes for a more gradual deformation and shape-ing.