You don't always have to. It just depends on the pattern. I have been buying charm packs for quite awhile and have always used them as is (at 5", without having to cut them smaller). That's just because the patterns I use call for 5" squares, not 4.5" squares. :)

I am thinking it has to do with math. Too many of my patterns have me trim them down

The Go Blog has videos how to use the Go and dies. Here are videos of everything you want to know about a Go:


www.accuquilt.com/tips-talk/videos.html

Same here. The few times I used charms I used them as is, no cutting down. But the patterns I was using was simple.

It just depends on the pattern. Some use smaller squares and some don't. I have seen patterns that use full size charms and some that use cut up charms.

I buy a lot of the charm packs. But I finally got the dies for the different sizes and now I won't buy them unless I really really like the fabric and don't have any that is a close match. I really love the Go! Baby.

That's why I like Connecting Threads for pre-cuts; they do a really good job on them. They're straight cuts (no pinking), no extra charge (costs same as the same amount of yardage would), and all of the charms, layers and strips I've bought have been extremely accurate. I've never had to trim down or square up any of their stuff. I just wish they had more fabric patterns that I like!

But even with perfectly cut charm squares, for the patterned mentioned in the OP they would still have to be cut down.

My shop chose the FOUR INCH size because it was the most efficient way to get a goodly number of cuts from 1 yard of fabric.
4 goes into 41", 42", 44" width ten times.
4 goes into 36" length nine times = 9x10 squares from one yard of fabric.

Four and one half seems "off" to me.

Here's some information from Moda on using 5" squares
http://www.unitednotions.com/measure...ck-cutting.pdf

And a quiltingboard thread about 5" squares
http://www.quiltingboard.com/main-f1...es-t55490.html