At what point do you spend the engineering dials/tokens? This question issue came up in my last game in relation to the Dominator title. If you have no shields, can you spend the engineering token prior to attack to spend them for the dominator extra dice? And, if you are at max shields, can you spend the shields for dominator and then recover them with engineering? I didn't see anything in the rules explaining when the command must be spent, only when it is revealed.

Timing wise a Token is the same as the Dial. Repair commands are resolved directly after revealing your dial.

Check the Reference Rules for the "Commands" Entry, Page 3:
"A ship can resolve the effect of a command by spending a command dial or command token with the matching icon at the appropriate time",
"Repair: Resolve after revealing the ship’s command dial."

So it happens before the attack part of your activation and indeed you could spend them to regain some shields to then use on your attack with the dominator title.

You can not spend the token or dial after the attack to regain Shields.