In theory not rounding the edges may be better with regard to flutter, but I have never seen or heard of anyone around here having a problem with rounded trailing edges. And they look a whole lot better.

I think taking care of other factors (no big gap in hinge line or loose servo horn/control arm connections, etc.) are a whole lot more important to preventing flutter than the shape of the trailing edge.

If you are just building something to throw around and don't care how it looks then leave them square. Otherwise I would round them as the plans describe and don't worry about it.