There is a difference between duck cloth and canvas, although the terms are often used interchangeably:
http://www.mothering.com/forum/66-ar...ck-fabric.html
I have duck cloth purchased as yardage from Walmart on my ironing board. To me, it is not canvas. My midarm machine came with canvas leaders, and that definitely is a much tighter weave than the duck. Duck fabric wrinkles terribly on first wash (and shrinks). I don't know what happens to canvas on first wash.
It appears to me that painting drop cloths could be made of either weave. Not sure that there is any way to know which ones you are getting if ordering online. My thinking is that the drop cloths sold in paint stores such as Sherwin Williams will likely be canvas and more expensive, as professional painters use them without washing and paint does not seep through the pores. What is purchased online may often simply be wide duck cloth, which would allow paint to seep through if not pre-washed and shrunk to close up the weave.