there some tip on this at stew-mac site, heres the link with photo , look at the middle of the page.
stew-mac
and heres is some quote from there :
Avoiding wood tear-out
Running a router around the entire guitar body in one pass is not a good idea; at various points, the cutter bit will face end grain from a direction that's likely to tear out wood. Wood tear-out is always possible when routing, but it's especially likely with long-grain tonewoods such as spruce tops. The solution to this problem is to cut the channel in a planned series of passes. Referring to Diagram A, first cut the four areas marked with arrows, cutting in the direction of the arrows. These are the spots where tear-out is most likely, and cutting in the direction shown minimizes the possibility.
(theres a picture in stew mac site here to demonstrate)
After making these four cuts, make one long cut around the entire guitar moving in the other direction, Diagram B. Because the first four areas are already cut, the router won't encounter likely tear-out spots on this long pass.