When you spend a command token, do you absolutely have to resolve its effect? For example, if I want to trigger engine techs on the Demolisher and spend a maneuver token, must I change my speed?

No.

You can resolve a command for no inherent effect.

Purely to say you resolved the command for purposes that trigger on command resolution.

Ok thanks. I thought that was the case, but I just wanted to make sure.

No you don't. You can spend any dial or token and choose not to use its effect. FAQ 4.1.1, page 4, last paragraph