Cutting fabric is an important part of quilting. I know it's not actually sewing, but accurate cutting greatly impacts how many tears will be shed during the piecing process. Obviously I'm kind of kidding, but the frustration of miscut fabric cannot be underestimated.
If you start your quilt with fabric just a bit off then your seams will be off and won't line up. Basically it's a snowball affect.
I remember when making my first quilt my friend told me, "Quilting is kind of like putting together a puzzle. Everything needs to fit and impacts the next piece." This has always stuck in my brain to remind me that each piece and block works with the whole quilt.
When I first learned to quilt I didn't really learn how to cut fabric correctly. I was working at a quilt shop and was cutting fabric for orders which doesn't need to be precise. So it wasn't unit later that a friend showed me the best way to get accurate cuts.
I thought I'd share a couple things that beginner quilters might now know (I didn't) which can make a big difference when cutting your fabric.
First, make sure your fabric is ironed. Sometimes the temptation is to be lazy and not take the time to iron fabric, however, ironing fabric can make a big difference and cause less frustration in your cutting. If you are having a hard time getting wrinkles out use some starch spray. I rarely use my starch spray, but when I do it really helps get the fabric nice and flat and makes it easy to cut.
Second, use a ruler longer than the width of fabric you are cutting. I made the mistake once when I was tight on cash of buying the cheaper of two rulers and instead of purchasing a 24 inch long ruler I bought an 18 inch long ruler. I was a new quilter and didn't even think about the fact that most fabrics measure around 42 to 43 inches in width and 21 inches when folded. So my 18 inch ruler was not long enough to cut my width of fabric cuts without moving it part way through the cut.
This meant it was hard to get a long even cut all the way through the fabric. Pay the extra couple dollars and get a 24 inch long ruler, you will use in probably every project.
Thirdly, measure and cut off your ruler, not the mat. This is the biggest mistake I made for a long time. I would line up the fabric and cut based on the cutting mat ruler. But I learned that cutting off the ruler measurements will give more accurate cuts, especially when you are cutting long strips like 2 1/2 in. x width of fabric. 
Fourth, keep your rotary cutter sharp. I like to get as much use out of my rotary cutter as possible, but to get a clean, accurate cut it's important to have a sharp blade.
Lastly, it's nice to be efficient and cut through a couple layers of fabric at a time. However, if you stack too many layers on top of each other then the cutting can become less accurate. I like to limit myself to cutting through around 3 fabrics. However, you decide what you are comfortable with.
If you are brand spanking new to quilting and have never used a rotary cutter then buy some cheap fabric in the clearance section of your local quilt shop and practice cutting.
The quickest way to become frustrated in a sewing project is with poorly cut fabric. So take the time cut it right, and remember we all make mistakes cutting. If you do, brush it off and start again.