I buy my needles at half off, so I get 10 needles for 5.99. the tension on your machine is better with a needle that is not dull, or bent.

I was always told dull needles are bad for the fabric

I change mine often. About every two or three projects that I work on. I buy them when they are half off, so they are really not that much. My thinking is why try and save a few pennys, when I have spent so much on my fabric. Kind of like wearing a dress from Nordstroms and shoes from Wal-mart.

I like this anaology!!! :) :) :) :)

It does make sense to use needles that are in good condition that are appropriate for the task.

Ones that have a rough tip can really mess up one's fabric

Or - like wearing a new (or new-looking) outfit with broken-down, scuffed shoes.

Needles should be changed every 8-10 hours of sewing, due to the fact that needle develop a burr on the tip. Think how many times is just one hour that needle has gone in and out of your fabric...a dull needle will punch a larger hole in your fabric, cause tension to be off, and on some fabrics catch on the fabric and cause a pull.
I was trying to get a bit more mileage out of a needle and the burr caught on the diaper that I was making a bit fancy and created a pull all the way across. I now have a fancy dust cloth...

If one sews "over" pins, but one manages to hit one of the pins with the machine needle, that messes up the needle point.

I change my needles as soon as I see any "problems" with the way they are working or start hearing the "clunking" noise. I buy them from www.cleanersupply.com. They are $1.75 for pack of 10 and shipping is between $3-4 so I stock up and buy 10 packs at a time. At only 21 cents/needle (including shipping costs) I can replace frequently without concern about money.

I change my needles only when I have to ie. break. I know. Not good.