Sure, it won't hurt anything. On the contrary, you'll be pleased to find that the groups shot will be always be significantly smaller than the accuracy measurement you've chosen to espouse would suggest they should be. It'll be like Christmas every time you measure a group.
Mean radius of a many shot group is better than the average of several few shot ones.
Group size is a convenient measurement and that's why it's used. There are certainly better ways to characterize accuracy.
I believe the military uses Mean Radius rather than extreme spread because they are less interested with how far the farthest two shots are from each other and more interested in how far the shots land, on average, from the aim point.