Could you please give me a very simple to understand explaination of what a scant 1/4" seam means? How do you achieve it? When would you need a "scant" one?

scant is really just a 1/4 inch seam, less a thread or two. Another explanation is JUST barely or ALMOST a 1/4 inch seam

You need it when your thread is heavy enough to 'move' the seam over a bit, or if your 'real' quarter inch really isn't. Even with a quarter inch foot, that quarter inch may vary from foot to toot, machine to machine.

Here's a great tutorial.
http://www.quiltingboard.com/t-89997-1.htm

it is definately worth your time to figure out where the scant 1/4" is on your machine. Your blocks will come out much better.

You need to sew "scant" in order for the turn-of-the-cloth in the seam. The "scant" part is how far from the raw edge your needle and thread are in order to achieve an "exact" 1/4-inch seam when you have ironed the seamed piece flat.

The degree of "scant" varies depending mostly on the size of the thread you are using for piecing. I just switched from a standard 50/3-ply thread to Aurifil 50/2-ply, which is a visibly thinner thread, and found that I can sew much closer to 1/4" from the raw edge to achieve an exact 1/4" after ironing. It's amazing how much thread size affects the turn-of-the-cloth in a seam!

Aurofil thread is my favorite thread!

So, it sounds like a scant 1/4" seam is just shy of 1/4" while you are sewing the seam, but once pressed it should measure 1/4"?

Yes!

However, it's not so much how the seam measures after pressing; what matters is how the block measures after pressing.

I just accessed the tutorial and it is very helpful. I understand much better how to check for and get a scant 1/4" seam. Do you just use the "scant" when the pattern calls for "scant", or for all 1/4" seams?

Also to consider ... I have found that it really doesn't make a difference unless you want to keep to the exact measurements - ie an 80 X 80 quilt WILL indeed be 80 X 80. If the end result doesn't have to be exact, then the most important thing is that ALL seams have the SAME seam allowance. This can take some adjusting from piece to piece if your using different types of fabric - or even different manufacturers. A Moda cotton will have a different end result than a Batik. Heck, even some Moda's are different than each other.

Also, you can probably still use your 1/4 foot guide rail - just move the needle position over to the left a smidge to accomplish the "final" 1/4" seam allowance after ironing.