Since there is no grid asset in SketchFlow, what is the best way to represent one? Does everyone just use the rectangle asset?
At the time the question was posed, there were workarounds and kludges. But now--with the advent of the just-released (June 7, 2010) SketchFlow/Blend version 4 there is a great solution: Sketchables from Philipp Sumi. This add-on to SketchFlow 4 lets you use concise markup to customize and tailor a control to display sophisticated output--including the SketchableDataGrid.
Sketchables is a completely new package, due to be released the week of June 21, 2010, according to his blog. He has a startling demo video on his site, though. For a snapshot of the SketchableDataGrid take a look at Figure 12 in my just-published article Creating WPF Prototypes with SketchFlow, June 18, 2010.
(I have no connection with Philipp Sumi other than admiration for his Sketchables product.:-)