I just decided to try out Autodesk sketchbook to see if I could do my pencils digitally. Sketchbook has a ruler tool that could definitely help with perspective drawing. So far I'm torn between Manga Studio's perspective rulers and just a plain old T-Square/Triangle solution. This is just me messing around with sketching a face in Sketchbook Pro and coloring the way I do in PS. It's okay. Maybe a little slow. Anyway... there you go.