Is there a known reason for the rounded toe & heel in US axes?
I'll say a couple things,for whatever it'd be worth...
To begin with,it's always very tough to try to compare two dissimilar tools,makes a guy go digging around further and further back in history of the tool's use,and possible original principles that may've influenced this or that feature.
A US-style DB evolved as a felling tool primarily.Quite heavy,and hung very long,it was a tool meant to be swung very wide,with lots of space around for that,primarily at a very big tree.
The cut itself was long and wide and ample,in every respect.
As such,the Point of Percussion could fall wherever it did-for the most part it'd still be within the cut.
I'm trying to imagine how a rounded heel or toe would act if the blade struck too close to the end of cut with either...and can't really see any problem,other than maybe a slight tendency to "roll" off...I think that'd be minor,and easy to control with a properly sharp axe.
Meanwhile,just like for any cutting tool,but especially the one swung so widely and forcibly,rounded corners are pretty much a must-it's a safety feature,preventing the hardened edge from forming a sharp corner,where it can easily chip because of insufficient mass.
Really,any radiused-edge tool,where the PoP stays close to the center of the blade,IS already "rounded" on heel and toe,as they don't really come into play too much.
IF they do contact,a rounded shape would make their release easier and more automatic,no point to dig in and possibly stop the entire tool,absorbing all the inertia in that necessarily weak sharp point....
That's how i picture a normal US double-bit:Great degree of radius to edge,very high,convex center-line,both heel and toe rounded and kept well away from banana grind for their own safety.
Conversely,something like a classic Kemi is a tool that was meant for much wider range of work(kinda notoriously a universal axe,no?).
The radius of the blade was not so steep,and there (normally)no convex profile to the blade-that,btw,gives the heel and toe more material.
Kemi was called upon to be used as a carpenter's axe as well as a felling(and often splitting)tool.
In bench-work the sharp toe can be useful,sometimes....Actually,judging by the wear pattern on older kirves the toe on them must've been used Lots,for whatever reason.
In sharpening a`non-convex blade one must follow the radius of the edge,too,so making a sharp point at heel and toe is not really optional,or they'd just end up blunt(which Would be a safety issue then).
Just casual thoughts here,more or less.