I looked through the tipping thread but could no find anything regarding this situation, so please forgive me if it was somewhere else and I did not find it.

So, recently I went to a restaurant with a few friends and had a unpleasant experience. The restaurant, of course, included a tip of 18% in the bill automatically as were a party of 6. But the service was rude and not very attentive, ergo absolutely not worth the 18% - we chose to give less and it got very unfriendly.

Here's my question: What would you do? In general, I find it too proactive to include the tip rightaway, but as long as the service is fine thats OK for me. But what if the service is not worth the amount the restaurant puts on the bill?

In another matter, I am never sure in the US (I moved here recently) if I am supposed to tip based on the pre-tax or post-tax amount. Also, at one restaurant, they added the tip automatically and then put tax on it - is that legal?