I have a book on paper piecing. It looks like I really do not need to cut the pieces a certain size but I just sew on the lines. Is the purpose of paper piecing to use scraps?

I use paper piecing to get perfect sharp points, you can do patterns you could not do with regular piecing, don't have to worry about seam size as you sew on the lines. Sometimes I use scraps, sometimes I cut pieces from yardage if don't have the right scrap.

Not necessarily. There are lots of patterns that you would buy yardage for. PP is a good use of scraps in some cases. The real reason why you paper piece is to create a block that you could not normally make with traditional piecing methods.

I'm a relative newbie to PP so no expert.
Yes you can use scraps ... though it depends on what size you have whether it'll work for you.

I find that I use a lot more fabric when PP than if I were cutting and traditional piecing a similar block.

Agree ... the bonus is the sharp points and accuracy along with the ability to do some patterns that otherwise I'd not do or would find just too darned fiddly!

make sure all the peices are 1/4 over each line.

I wouldn't attempt paper piecing without the add a quarter ruler. It's a must-have tool, IMHO. Here's a video demonstration. I have all three sizes, and they are indispensible.

http://m.youtube.com/#/watch?v=Xv2gKMx9CCM

check out some tutorials on the internet. i love PP and am self taught.

I love to pp. Cutting the material for a quilt is the one thing I dont like to do. With pp you dont need to be so precise. I usally make my seams a little bigger and cut them down to 1/4 inch. Sometimes I buy specific material and sometimes I use scaps. I love the way you always get perfect points. Blocks turn out really nice.
Have fun!!

I'm doing my chicken pin cushions by paper piecing.

Just my opinion, but I think any ruler you have with a 1/4" measurement from the edge works. I know the 'add a quarter' ruler has a thin lip to go over the folded paper but I think the same results can be achieved with any ruler with 1/4 markings.

But, yes, pp'ing is the hot tip for precision, particularly with points.