When my bike was arrived, the headset came with some spacers below and one spacer above.
My bike
Is it safe to raise the headset all the way with no spacer above it?
I don't want to flip the stem yet for astethic reason.

Trek advises to place spacers above and below the headset:
velonews

Not sure if that rule applies to aluminum steerer.

I believe you are talking about the *stem*, not the headset.
If the steerer is carbon fiber, you want to keep a spacer above the stem.

Thank you.
So my stem with aluminum steerer should be ok.