ive experimented with this in the sim (738), basically anything lower than 100ft is regarded as alittle unsafe even in perfect conditions, and for some reason i have the tendancy to look outside for a few moments rather than scanning, wired. also engaging the AP at this level results in more fluctuation of the pitch angle on climbout.
Personally i have found enganging over 400ft allows for a stable climb out, like people has said above.
G-STAW