There is definitely an ergonomic way to sit at the sewing machine. Here is a link to OSHA recommendations for sewing stations:
http://www.osha.gov/SLTC/etools/sewi...iondesign.html
Height is probably the biggest consideration, as the wrong height will put considerable stress on the shoulders.
However, also consider if you are sewing centered on the machine. The needle should be directly in front of you, not the front of the machine. A lot of sewing machine cabinets do not take that into consideration, making you sit too far to the right of the needle.
Another consideration is the distance between the cabinet's edge and the needle. For quilting, it is usually better to have that distance larger than normal. A table (say, from Ikea) can be modified with a cut-out and drop-down to hold a machine at the ideal distance from the front.