Here is my small suggestion for transparent text clip and PNG transparent images.
The default background for transparent clip should be dark-and-light gray checked pattern as it is in Gimp or Photoshop. It is kind of visual standard.
The current situation is that black solid color is used or the current position "timeline snapshot" (current frame picture) is used. This may be distracting to users. My suggestion is not to show timeline snapshot unless user explicitly wants to see it.
For already placed text clips the default snapshot position should be the first used position snapshot (not current position) and ideally list of used positions available (on timeline marks or simple popup menu list, whatever) PLUS remove text clip from composition of snapshot (if such is visible at that moment) as it makes text looks "doubled".
Plus there is a bug in text position (see my bug report - text appears in different location when in property window and different when rendered in Timeline Monitor) Until this is fixed it is highly recommended to use more safe "neutral" pattern background then to expose the bug in the "Full Monty".
Until this bug is fixed I would suggest not to show snapshot at all.