that is what my accountant told me about charity work too, you can't write off what it would have been. Your expenses, are just the payrol, fuel, whatever the job ate up that day doing the job, like normal.
We will donate to churches, sometimes police dept and work on champion trees for free or greatly reduced price. I have lots of tree removals for my church coming up, wrote up the regular bill and cut it in half by 50%.
Yes, I agree with someone earlier that said, watch out, if you do something for free, something is likely to go wrong. But for me that usually happens on a regular job, where the customer adds something that day and I said, "no problem, I'll just take care of that no charge", and it becomes a problem or more work than thought and i want to kick myself.