Agreed, I don't think there is one ideal stance for both boxing and muay thai, it really depends on what suits you best. Really, you should be able to make minor adjustments to your stance as necessary for the given opponent. Truth is you can make that narrow lead shoulder boxing type stance effective in kick-boxing as well, you just have to learn to throw kicks by circling to find the angles vs. coming straight in and transitioning to kicking is a more natural transition if you are more squared up. As Ambertech says "there are advantages and disadvantages to both" you just have to know what your greatest advantages and strengths are and use the stance that best lets you leverage those advantages against your opponent. That said, you have to be able to make the adjustments necessary to properly defend yourself as well.
I personally will use a low lead hand, narrow shoulders boxing stance in MT or Kickboxing sparring against certain opponents (usually, shorter stockier guys who like to get inside), and it is often very effective...... being i am a tall and long fighter with quick enough feet, it lets me take advantage of my reach and keep pressure guys on the outside with my jab.... That said, a guy with quicker feet or that i know throws good high kicks, I would definitely square up more and get the lead hand up higher. Just depends, but you can make many stances effective, the key is your mobility in that particular stance and your ability to make adjustments when necessary.