The first Campaign mission I played, when I set up, I put tokens in the spaces where the Imperials spawn, including the yellow and red spawn points.

This was wrong, wasn't it?

Since then I've been playing without revealing these locations, but I want to make sure I've been doing it right, since I just saw pictures of someone playing with each spawn point revealed via token like I did my first game.

I believe they are hidden since there are no tokens to represent the spawn points.

No token needed. Imperial player is only one that knows.

would spoil the surprise if they knew where we was coming from

I got it wrong too, thought the circular Imperial tokens with the colors where spawn locations, but turns out those are just a type of an objective token for the Imperials. The only spawn point you put down is the blue-ish Rebel one, which counts as the 'entrance'. This got me real confused sometimes, because there is at least one mission that said to spawn stuff at the 'entrance'.