I not really need it now, it was just an idea ;)
I could also use a clear font in the sketch design so i will not have the problem. So it's not really an important thing :D
But i think it does look better if a handwritten like font is used in a sketch design. WireframeSketcher supports colours and my idea is
Start with a sketchy look wireframe that looks mostly black and white (only use colors to bring something into focus).
When the wireframe is ready, start to bring colour in its life. To get a look that is more "final" the clear style is better than the sketchy one. And in a clean style i would use a clean font.