The most important reason to index plugs is to prevent the piston dome from contacting the ground strap on the plug. If you don't need to do it, it isn't worth the effort. It usually doesn't matter if the ground strap is up, toward the front, or toward the rear, just not down.
I use a sharpy to draw a line on the porclain on the ground strap side so I can see the orientation with the plug installed & tight. It may take a bunch of plugs to get a full set indexed without index washers........
Johnny