I think it kind of depends on the look you want to achieve. I can comment more on the sketches rather than the canvas work.
Tablets are definitely great tools to have and will often come in use for graphic design - but I don't think they can replace pen and paper 100% of the time.
When I want to turn a pencil sketch into something digital, after scanning it in (300dpi or more) I go about things one of two ways. I either,
A) Import the sketched drawing directly into Illustrator or Flash (I actually prefer Flash for this even though it isn't really a graphics program) and then go about the time consuming process of actually tracing all my lines - and I actually will OUTLINE each pencil line, then go back after and use the paintbucket to fill the lines in. It takes time, especially for complex drawings, but the result are easily worth it. Other thing I do is,
B) Run the sketched drawing through Photoshop first. I usually start by cropping and positioning the sketch the way I want and then, with the graphic selected, press CTRL + L. This allows you to adjust the white/black levels of the artwork. Drag the black/gray/white tabs closer together somewhere along the scale and you eventually will have a straight black and white (no gray) version of your drawing. At this point, either leave it as is, apply filters or export to Illustrator.
Once in Illustrator, you can select your graphic and use the Live Trace button to convert the graphic into vectors. If you click the little drop down arrow at the side of the button, it gives you lots of different tracing options, so have fun and experiment with them.