Pretreating Fabrics

This is something I still hear being argued about amongst sewers, and it needs to stop.  Let this be law:
ALL FABRICS MUST BE PRETREATED.
ALL. 
NOT SOME, ALL.
When fabric is being transported from overseas, it's coated in a variety of chemicals to help it survive the journey intact.  Formaldehyde is a common one, it keeps the bugs off the fabrics just as well as it preserves dead bodies.  Various pesticides are used, too, because bugs find fabric tasty.  Understand that a bolt may spend weeks, if not months, on a cargo ship and being stored in a warehouse before hitting the shelves at your local stores.  The chemicals are a necessity to keep fabric from being chewed up and keep foreign diseases out.  You may notice that some fabric stores are kind enough to post warnings that fabrics are treated with chemicals known to cause cancer.  Treat that warning as applying to ALL fabrics, even if no such warning is present.  You don’t want your kid wearing that stuff, let alone chewing on it.
Excess and loose dye is another reason you need to pretreat your fabrics.  I’ve seen the water turn black as oil when soaking fabrics.  You do not want that mixing in with your other colors when you later go to wash the finished garment.  It’s best to get it all out before working with the fabric rather than having a nasty surprise after.  
Of course, there's the problem with shrinkage.  Even washing with cold water and line drying I've experience shrinkage in fabric.  How much a fabric will shrink will depend on the material itself and how it is washed.  You'll never know until you wash it, either.  If you're getting fabric and measure your pattern out edge to edge with no room for error, you might want to get a few extra inches just in case.  I've lost up to an inch per yard on all sides the cut with certain cottons.  That would be a devastating loss to a finished project.  It's best to take care of shrinkage before even starting on a project.
So that's the why, now the how.  Naturally, it depends on the fabric itself to determine how to pretreat it.  The best rule to follow is to pretreat it like you would wash it normally.  The bolt of fabric will give you washing instructions.  Jot them down or snap a quick photo with your cell phone.  This should be your starting point, but there's always room for experimenting.

My methods are pretty simple.  For everything but wool (cottons, synthetics, blends, etc.), I soak it in a bucket of warm water and Oxi-Clean for a couple of hours, agitating it every once in a while, and then run it through a wash cycle in the washing machine on its gentlest mode using my usual detergent.  Wools, I still soak in a bucket but skip the Oxi-Clean as it's supposed to kill wool, though I've never tried it myself.  Everything I hang dry.
Oxiclean is AMAZING.  Believe in the word of Billy Mays.  Chew on this, here's some fabric I soaked for two hours in it.  Looks exactly the same as when I put it in, no fading at all.

Now look at the bucket of water and what this fabric left:

Disgusting, right?  It's pure black.  Two gallons of water in a five gallon bucket, maybe four inches in height and I can't even see the bottom, let along an inch or so in.  It soaked for two hours, but when I went back out 15 minutes after dunking the fabric, it had already turned this black color.  Over the course of that two hours I would squeeze and agitate the fabric to make sure it all comes out.  Do you need anymore proof for pretreating
Just wanted to say, this isn't that much to worry about.  That black water isn't pure cancer causing elements.  Most of what's going to come out of the the fabric is loose dye, which is why the water is black.  I do my whites the same way and the water comes out clear, but like I said, you never know what is on that fabric so pretreat it no matter what!
And yes, I did say I put wools in the washing machine!  I find that you can get away with machine washing many supposedly ‘dry clean only’ fabrics, such as wool as long as you are very gentle with them AND DON'T PUT THEM IN THE DRYER.  Wool is generally not listed as machine washable because when rubbed, it starts to 'pill,' which gives it a fuzzy look.  Your mileage may vary, so when not sure, just cut a small swatch out and test it in various ways.  That’s how I learned about wool handling the washer.
-Cat