25 years as design engineer for a utility, so I know of what I speak.  30 ft pole is set 5.5 ft deep each extra 5 foot of length requires and extra 0.5 ft of depth.  Corners and deadends are not set any deeper than normal.  Refer to the push guy use of another pole in posting above.    Edit:  I should also note that not having guy wires doesn't cause the pole to lean over.  Usually the pole will bend above the ground line until over a period of time it looks like a parenthesis.

Some guys just like alot of overkill in their construction and that is fine, as long as I'm not the one doing the extra work.