Steeking is very important, on a traditional woollen tartan it isessential to hold the pleats in line, stopping them drop and maintaing the "tartan line".On a modern style kilt where the fabric is more normally a more rigid one like canvas or drill, with many modern kiltmakers stitching the edges of the pleats as well, there is less fabric distortion than there is on wool. But I always steek whichever style of kilt I make,admitedly a little more thoroughly with a tradtional one. I think steeking on the inside so that it almost invisible is the way I prefer, although some makers do have a visible line of stitching through the layers which performs the same function.
But yes, steeking is important, and a sign of a well made kilt.