I just did some research and finding some cool links for all of us:
1.
http://sewing.about.com/od/sewingmac...roubleshot.htm
2. found these tips:
A good rule of thumb is to change with every project but I'm cheap and wait for these tell-tale signs:
1. A popping sound whenever the needle hits the fabric.
2. When bobbin thread tangles or skips a stitch.
3. When the stitches start to look uneven or pop up (just plain ugly).
3. lots of great information on this site
http://sewing.about.com/od/sewingmac...chine-Needles/