In short, yes it is a good idea! If you are sewing a new pattern for the first time then I really would encourage you to cut the pattern in muslin first to make a toile. This will allow you to try on the toile to check the fit before making it in fabric.
Let’s explain the terminology first. The term ‘muslin’ means – a lightweight cotton cloth in a plain weave. In the fashion industry the term ‘muslin’ is commonly used to refer to a ‘practice garment’. This is also commonly referred to in french as a “toile”. The definition of a ‘toile’ is – an early version of a finished garment made up in cheap material so that the design can be tested and perfected. It can also mean – a translucent linen or cotton fabric, used for making clothes. In short both of these words can refer to the fabric itself or a test garment.
So whether you call it a muslin or a toile, it is very useful to make up a new pattern in a practice fabric as a test garment. Most importantly it allows you to check the fit of the pattern, but is also helpful in reviewing the construction of the garment. Not all toiles are made up in muslin fabric. Muslin is a common choice of fabric for practice garments because it is so inexpensive, but you can use any cheap fabric so long as it has a similar weight, feel, and stretch to the fabric you ultimately plan to make the garment up in.
The good news is you do not have to spend time finishing the seams allowances and you can omit such sewing details as top stitching, facings, and buttons. Sew the garment using the correct seam allowances stated on the pattern to check for fit, fullness, and ease.
It is not easy to fit a muslin or toile on yourself. You may need a friend to help you. Check the fit to see if there is too much or too little ease, whether the fullness is appropriate for your body type, whether the waistline, shoulder seams, and side seams sit on your body correctly, if the length in the body and sleeves need to be adjusted, and look at the armholes and neckline to see if they are where you want them to sit. If you find that you need to make adjustments to the pattern, mark these adjustments right on the muslin, using a marker or dark pencil. When you take the muslin off you will have something to refer to while you make the adjustments to the pattern.
If you make just a few minor adjustments, then after correcting the pattern, you should be able to cut into your fabric. However, if you have made major adjustments to the pattern I would recommend making a new toile to check these new adjustments.
If you like the pattern and plan on making it again in the future I would suggest that you retrace the adjusted pattern onto oak tag, sweddish tracing paper, or even a regular tracing paper to preserve the changes that you made.
We all come in different shapes and sizes, and one pattern is not going to fit us all the same. It is worth the time it takes to make up a muslin. In the end it saves a lot of disappointment and frustration when your final project fits like a glove and looks spectacular on you!