Help with cropping

Hello all and thanks for your previouse support. Todays topic is cropping.

Here is what I would like to learn, I have a 40D so the crop factor is 2X3, so my problem is how do I shoot for 5X7s and 8X10 with full body shots that I have to crop?
I will have an upcoming shoot, (MY first of its kind) where I will be taking full body shots of dancers. Of coarse my packages will include 5X7 and 8X10s .
So do I shoot with frame left on both top and bottom, just top, just bottom? How much frame do I not fill?
I just want to know if there is a magical way that you peeps here on the forum plan for crops other then your native camera crop.

In my mind (I am about to start test shoots this week to prep) I should leave more frame empty on th top but will I run into any centering issue?

I see no reason to err to the top or bottom particularly, I'd just keep it central, and loose.

Some vaguely relevant stuff here: Cropping and Printing: The 8x10 Problem

If you know you need to print 8x10s just make sure you don't fill the long part of the frame. Alternately don't offer 8x10s, offer 8x12s.

So I should offer 4x6s and 8x12s?
I just think people expect 5X7 and 8X10s so how do i explain the size difference without them thinking I somehow gave them something of lesser value?

6x9s as well, maybe?

I offer all sizes, or custom sizes, my shopping cart gives a crop preview which explains things pretty well. If people choose a size that doesn't suit an image I just tell them and ask if they'd like to go smaller or larger.

I'm pretty sure Canon makes an insert with lines to see what the crop will look like. It replaces the "screen" that comes with the camera. I forget what it's called right now.