Center vs. Approach Altitudes

Hello - I have been looking in the AIM and a few other places and am
trying to track down where enroute approach control end and Center
control altitudes begin. When planning arrivals and depatures through
NYC/DC/PHL and BOS airspace I was curious where those altitudes begin.
I've been told several times that you need to be at 5000 when passing
through those airspaces IFR in Cherokees, etc and and was curious if
anyone knew where the center altitudes start since it would be more
economic on most flights to be higher [unless the winds are howling].
It is just as easy to plan 11000 or 10000 as 9 or 8 for a 350-450nm
leg.

I guess if no one knows, Ill just have to ask next time!

Joe Farrell
N8846J