Do I really need a special ruler to...
 

make the corner piece for a quilt set on point? Really?

No. There are plenty of online charts to tell you how to cut it.

Here's a chart for setting quilts on-point. Hope this helps.
http://quiltbug.com/articles/on-point.htm

The CORNER TRIANGLE is just a big HST.

The SETTING TRIANGLE (at the end of the on-point row) is just a big QST .

Both units are cut from squares (unless the fabric is directional/striped and you want to control the flow of the print design).

you can also calculate your own and use any square ruler...measure the length of the side of the on=point block that needs a corner....let's say it measures 10 3/4, or 10.75. (don't forget seam allowances). Use a calculator (unless you have one of those wonderful brains that can do this in your head) and divide that number by the quilter's magic number - 1.414 - and you get the length of one side of a square you need for two setting corners. 10.75 div by 1.414 = 7.6, which is between one half (.5) and 5/8's (.625) so I'd cut a square 7 5/8th's, cut it diagonally in half and you have two corner setting triangles for your corner block that measures 10 3/4 on the side. This gives you a little to trim off....cut a second square this size and you have another two HST's to finish the quilt...and the sides are on the grain. You can trim it off after you stitch if you finger press then match the center point of the block and the triangle - since the diagonal is on the bias, make sure not to stretch, and that you have equal amounts hanging off each end before you sew.

I have the super large 30" set in triangle ruler and it saves me a lot of figuring. I can just cut the size of the set in piece according to the size of the blocks. I don't need it but I don't need 95% of the quilting items I buy. I just want them.

Quiltville.com has a chart that printed off. I'm on my smart phone so I can't copy the link.

nope, but they work great ;-)

The rulers/tools make it easier to cut those triangles out of strip of fabric, not cutting a big square (requiring more fabric)

Not a 'need' but a nice addition. You will probably use the ruler frequently once you learn the use. It makes the cutting and piecing go much quicker. I love mine.

Then if you can use a coupon ... well, what a deal!