I've been told that patience is a virtue, but it can be so hard to resist cutting into a new fabric when it finally arrives.

Ordering fabric online can be like waiting for Christmas. You know that there'll be a special present just for you. You even know what sewing pattern to use with it and you have made sure to have matching threads on hand. Then, when Santa Claus the mailman finally comes, you rip open the parcel and do a little dance of joy. You start to take out everything you need for your new project: the pattern, scissors, thread... but oh no!

It dawns on you that the fabric hasn't been washed and you were so looking forward to getting started with sewing your new garment. You have already waited terribly long (waiting for something good can feel like an eternity), so you begin to wonder if you really have to wash your new fabric before sewing.

There are 3 good reasons to wash fabric:
1- it might change in size when washed
2- it might bleed colour into other fabrics
3- it could contain unwanted chemicals

Getting rid of unwanted chemicals might not be a reason to wash before you sew - but it is a good reason to wash before you wear them. Especially if what you sew is going to be used by small children. In some cases, fabrics are treated with stuff that could cause skin irritation, and it's always better to be safe than itchy.

Now, let's look at the issue with the fabric changing in size when washed.
Natural fibres (e.g. cotton and linen) will most likely shrink and synthetics not so much. Some fabrics will grow a bit in one direction and some will crinkle (e.g. muslin/gauze) when washed.

If you are making a one-size-fits-all project out of only one fabric, then you can probably get away without washing it first. Keep in mind: if there are a lot of pattern pieces you could risk getting some puckering here and there where corners meet. If you are putting in a zipper, you definitely want to make sure that the fabric doesn't shrink afterwards. It would also be very sad if you can't fit into your amazing new dress if it shrinks in the first wash.

An important rule of thumb is to wash the fabric in the way you plan to wash your finished project. You should, of course, always take a look at the washing recommendation that comes with the fabric. But let's be realistic: if you make children's clothes you're not going to be handwashing them in cold water, right?

How to best prewash your fabrics
Some fabrics fray at the edges when you wash them. It's, therefore, a good idea to zigzag or overlock the raw edges before throwing them into the wash. Knits like jersey and interlock tend not to fray much so you might not need to sew the edges on those.
It's also a good idea to wash similar colours together if you aren't sure if the fabrics will bleed onto each other.
Using a tumble dryer is hard on your fabrics (also not very eco-friendly), so you should consider hanging your new fabric out to dry. On the other hand, If you plan to sew something that is going to be tumble-dried later, then it would actually  be a good idea to tumble dry it on the setting you plan to use.

Then there is the issue about colourfastness.
You don't want your new fabric to bleed colour into other fabrics. Even if you wash the finished garment separately in the first wash after it's finished, you need to consider that the fabric could bleed into other fabrics in the garment, like for example a white neckline trim.

Testing for colour fastness
If you have bought lots of fabrics in different colours, and you don't want to wash them separately, you can try to do a little test on some swatches.
You can dampen a white cloth made of natural fibres, like cotton or linen, and rub it on the fabrics suspected of bleeding. If the colour rubs off, they should be washed separately to get rid of the excess colour.
If it doesn't rub off and you are still unsure, you can cut off a little swatch of the fabric and hand wash it with some detergent in a bowl together with a piece of white cotton or linen fabric to see if the white cloth stays white.