Most beginners quilting articles I have read usually prescribe pre-washing fabric, but I'm going to go out on a limb and say that I have never pre-washed fabric and I have never had any problems as a result.
Somewhere once I did read something that said modern cotton fabrics no-longer needed to be pre-washed, but that's the only contradictory view I have read. Far more important to me is to pre-wash your cotton wadding, and yet I hardly ever see that advised.
Yes, some people love the wrinkled antique look, but I'm sure I'm not alone in being disappointed when I found the intended look of a quilt altered after first wash. I now pre-wash for shrinkage every time.
I've lived in some pretty remote places and my thread supply has been quite unreliable at times, so I've often had to resort to whatever I can lay my hands on. I learned the hard way that cheap thread breaks or splits causing knots or repeated re-threading of your needle. I now make a rule to only use Gütermann thread and I have purged my thread stocks and removed everything else.
This sounds kind of obvious, yet most new quilters roll their eyes when you tell them they need to buy another presser foot (what another expense?) and the truth is you can get away with using the standard foot for quite a long time on simple designs, until you use a more complicated pattern. Then suddenly the small difference in seam allowance is enough to mean a design won't match up. Another one I learned the hard way when I made the assumption my standard foot was 1/4 inch (it isn't!)
This is actually worth repeating a few times till it sinks in, because when I first learned to quilt I actually ended up a little terrified about making mistakes. Quilts are handmade (whether with the aid of a machine or not) they are not mass-produced items. Do your best, and of course unpick if you make mistakes, but don't get hung up if things end up a little wonky, it's part of the charm.
So now it's over to you! I'm sure you probably disagree with at LEAST one of those points, so what advice would you give to first time quilters?