After doing this for way longer than most, I used to use the #16 X-Acto for all the points Pat has mentioned, however I switched over to the X3209 X-ACTO retractable knife and find the small angled blade to be just right!
Kind of a shorter version of a #16 blade, and being retractable it eliminates a lot of dangers of a sharp blade that uis exposed all the time and the pocket clip keeps it from rolling off the table.
You really have to try it to realize how well it works, also keep one in my on-site install tool kit!
http://xacto.com/products/cutting-solutions/knives/detail/X3209