As you have already said, I thought the flanks were composed of the "better" grenadiers and light infantry troops.
That leaves the line infantry to the centre. That is, don't think; shoot, reload, shoot, reload, repeat etc.
I guess the difference is purely a distinction in their uniform and as you are a non button counter i would assume the differences could be ignored.
In saying that, having the right looking uniforms would probably enhance your game - Napoleonics is a dangerous, dangerous wargaming era I thinks!
The Highlanders would have had flank (grenadiers and light) and centre (line) companies as well, so again its just your choice, to button count or not.
I'm sure someone else has a lot more knowledge than me though.