I usually use an 11 or 12 to piece but a 16 or 18 to machine quilt. I FMQ all of my quilts and I like to use decorative threads and you really need to think about using a larger needle when you have a 35 wt thread. Also, I was just at a quilt weekend and a question came up from someone about when to change the needle. One girl had gotten her new machine in early summer and had not changed the needle. She does a lot of piecing. We told her that the weird thumping noise was her sewing machine forcing a dull needle though multiple layers of fabric. She changed the needle, the thumping stopped and she said she thought the machine was going faster....why wouldn't it...it wasn't working as hard pushing the needle into the fabric. We agreed that 8 hours of quilting or piecing was enough wear and tear for one needle....throw it out, put in a new one and go on your way.