Here is a great paragraph from the Crane website
It is the exception, rather than the rule, that a cam may be out of phase, but this should be established to insure an accurate performance baseline or point of tune.
WHY DEGREE-IN YOUR CAM
The purpose of degreeing a cam is to insure that the cam is phased correctly with the crankshaft, per the cam manufacturers’ specifications. Some factors that may cause improper phasing are:
1. Cam or crank gears are incorrectly marked.
2. Keyways are out of position on gears.
3. Keyway in the crankshaft is misindexed.
4. Cam dowel pin or keyway is misindexed.
Here are some great tutorials on degreeing cams from the major manufacturers. They are all just a little bit different.
Crane
http://www.cranecams.com/instruction...ing/degree.htm
Lunati
http://www.holley.com/HiOctn/TechSer...IECCTech8.html
Isky
http://www.iskycams.com/degreeing.html
Comp
http://www.compcams.com/catalog/022.html
http://www.compcams.com/catalog/023.html
Hope this info helps =)
Andy