Originally Posted by
mstateglfr
No spacer above means the stem is tightened and the top of the steerer tube is impacted by the tightened stem. That seems less than ideal.
Setting it up so the entire stem is below the top of the steerer tube means the stems full clamping force is completely on the steerer tube and none of it is pinching and pushing in the top of the steerer tube.
If we want to talk about structural integrity, the area where the bolts are must be supported adequately from inside by a compression plug for everything to be ideal. The reason why it's safer to recommend everyone leave a 5mm spacer are compression plugs like this one:
Not ideal
You'll notice the top (black) part of the compression plug doesn't actually expand, so without a spacer on top, the top bolt of the stem will, depending on location, clamp a partially unsupported bit. This type of steerer plug is also why a stack of spacers above is unwise if you haven't checked what sort of compression plug you've got; it's really short, so if you put 20mm of spacers above, the bottom bolt will clamp an unsupported bit. It's not a very good design, but it gets used.
This one is completely fine structurally without a spacer on top, and is longer meaning that with a spacer or two on top you'll still be fine nevertheless:
A much better design.