* Disclaimer: Please check your machine manual before using needles other than those recommended by the manufacturer!
It's not just puckered seams, broken threads and skipped stitches, the wrong needle can leave holes in your fabric, throw off your machine timing, and even damage your bobbin.
There was a time I thought a needle is a needle is a needle. Now, I know better, and my sewing has improved just by ensuring I use the correct needle for my project and change them when required.
The European needle sizing system is based on the diameter of the needle shaft just above the eye, whereas the American system uses an arbitrary number. Needle packaging generally shows both numbers for example 60/8 or 70/10 - the larger the numbers, the larger the needle, and accordingly, the finer the fabric = the finer the needle; the heavier the fabric = the heavier the needle.
As different types of fabric are made in a predictable weight (chiffon is light, denim is heavy), you can usually make an educated guess as to the best size of needle to use for each project. To make the decision easier, you can refer to the following chart, which not only shows the recommended needle size for different fabrics, but also the names given to the needles, which is determined by the characteristics of the needle point (sharp, rounded etc).
Obviously a needle needs to be replaced if it gets bent or broken while sewing, but outside of this, the general rule is that a needle should provide 8-12 hours of sewing before it needs to be replaced.
While this may seem excessive, think about the last time you used a brand new blade on your rotary cutter, or a freshly sharpened pair of scissors - so much less effort is required to get through the fabric, and it's the same for your machine, it needs to work harder when using a dull needle. If you're lucky enough to get even half an hour of sewing in each day, that optimum 12 hours of sewing time is equivalent to almost a month of sewing!
Resolve to do what I do - change your machine needle on the 1st of the month and enjoy having a fresh, sharp needle working for you each and every time you sew.