I too wonder about this stuff. I know LL owners dont like to talk about mirroring...but its like...everyone does it and it would be nice if there was some type of uniform rule...
Some LL's are cool with index1 index2 index 3
Others need their own directory but allow the directory to be named 1,2,3,4 etc.
And then there is a whole different group who need their own directory and it must look like its not mirrored....in which case people use keywords...
I of course NEVER mirror my sites
But if i was going to, i'd do the 3rd option just to be safe....