Traditionally artillery are on large rectangular bases. This is how everyone else I know has always mounted their artillery stands. It is just the way it was done. A gun, and three to five figures and that is a artillery stand. Add a limber and you are done.
But, on the Fife and Drum miniatures blog I noticed that one player mounted his artillery on a round base. I cannot tell you why, but it looked great. Better then great, I really liked it a lot. So much so that I mounted my War of 1812 artillery on round bases.
Now I am redoing my American Revolution collection. Not a really big deal. Just time consuming. Stay tuned for more pictures when done.