We are planning on making the surround out of granite, and our builder is telling us that it is both "too difficult" to redo the front of the firebox with an arch and that it would be perfectly fine for us to create an arched opening by having the granite cover the upper corners of the rectangular firebox.

This seems suspicious to us... it would expose the back of the granite directly to the heat from the fire.

We're unsure if that would lead to risks, such as transmitting heat directly to the surround (without it being obvious that the granite is directly exposed to heat).

This is a 42" masonry fireplace built to "standard" dimensions (so we've been told).

Do we need to have the mason redo the work and build in an actual arch in front of the firebox?
Or are we overly sensitive to the impact of having the surround partially cover the firebox, meaning it is perfectly ok to go with the builder's recommendation?

Been lurking here getting other information to use in our build (thank you, Garden Web Forum!)... hoping to get some insight on this fireplace issue, as there doesn't appear to be much information about this that we could find on the web.
Thanks in advance for your help!

Well, what we decided to do along with the builder and mason was to patch in firebrick in an arch pattern under the steel lintel. While one could still argue that it isn't the "right solution," it will suffice for our needs.
Basically, the rectangular box has been transformed into an arched opening with the "arched" part not being load bearing at all.
Is it what we would have wanted? Not exactly.
But, it safely answers the question of how to protect the back of the granite from any direct heat/smoke for the exposed corners.