Well, you don't want the tang to be exposed. So use your tracing of the tang to make sure that doesn't happen.
Otherwise, make it comfortable to hold. Round can roll in your hand. Oval or rectangle with rounded edges won't roll as much.
A good way to get a nice radius is to make a flat 45º bevel off the 90º corner. Then take the 2 corners off that. Kind of like planing a square board into a round board.
Start with 4 sides, take off the 4 corners and have 8 sides, take off the 8 corners and it is almost round.
Here's a photo, the bevel is sanded in on the left, and the pencil marks show on the top.
....and finished....
I like to use 80 grit sanding belts on a big file. Secure the top with a "bunny" clip and hold on to the bottom.
Knife in the vice. Gives you plenty of control.
Tape the knife blade and remove it from the vice when you walk away.
A knife in a vice is a dangerous thing, taped or not.
Above after glue up, below after doing the bevel thing and the final smoothing with the old shoe shine method.
If you start with the shoe shine, you lose a lot of control and tend to chase symmetry.... at least I did.
Hope this helps!