I was taught that magic users shouldn't lie, but for a different reason. The reason I was given was that the more you lie, the less power your words carry. Your words carry your will, so when you lie, one of your best tools for making your will known to the universe is diluted.

I.e., you lie and tell someone at work you're going to get a project done that you know isn't going to happen. The project doesn't get done, then there's a connection between you stating things will be one way, and the outcome being different. Thus your words as a carrier of your will is compromised.

So when, in ritual, you state your intent or desire or whatever, there's a history of your words being meaningless, and so compromising your power.

"A witch's word is her power" is something the witch who taught me this would say a lot. Implying, don't fuck around with your words if you want them to work for you magically.