Cannot disable custom thumbnail for field

A client has a situation where they cannot disable the custom thumbnail for a Embedded Video Field.

We had already tried the method here (http://drupal.org/node/227430) but that disables custom thumbnails altogether, and you should be able to turn if off on a per field basis.

Digging around I found the following worked but would like someone to verify this is the right to do it :-).

In emfield/contrib/emthumb/emthumb.module v 1.7.2.10, line 301:

function emthumb_emfield_widget_extra(&$form, &$form_state, $field, $items, $delta = 0, $module) {
// Add these two lines at the start of the function
if (! $field['widget']['emthumb'])
return NULL;