Joseph D. Farrell know what to add wrote:
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.

You won't find that information in the AIM, and probably not in any
place easily accessable to pilots. Every approach control is different.

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've given up trying to second guess traffic patterns and altitudes.
File whatever you want, and you'll get whatever they want to give you.
Traffic flow into a congested area like New York changes depending on
which approaches are in use. The traffic is where the traffic is; it
doesn't really matter if TRACON or Center is controlling it.