Could be different. The min insertion of the post has as much to do with the leverage the post puts on the frame as it does with any issues related to breaking/damaging the post. Too much post sticking out w/o enough in the frame equals problems.
Just ask my buddy with the top tube (yes top tube) with stress cracks on his Ti frame from having less than the min amount of post in the frame. The post (Thomson I think) however is fine.
My stock seatpost is 220mm. Ye, from the bottom it is 70mm (about 3 inch). It is pretty short for me so i plan to get a longer one. i just wonder the additional length is really 80mm if I get a 300mm seatpost. That's why I ask the length of the min insertion mark are all the same. Should I get 300mm or 350mm? i'm 5 feet 11. My bike has rear suspension, so cant post sticking in too much.
I have three seatposts of different lengths from different makers in front of me here at home.
Aluminum, 300mm, insertion 100mm from bottom
Steel, 360mm, insertion 80mm from bottom
Aluminum, 400mm, insertion 100mm from bottom
From this very small sample, it seems that insertion point might vary from post to post depending on lengths, materials (as evenslower hinted at), or the whims of the manufacturer. You should probably email the manufacturers of any posts you're interested in to ask them for specs.