General Tipping on Wine
It is customary in the US to tip on the wine at the same rate that you would for the entire meal. Just as you would if you were drinking cocktails. So, if you are planning to leave 20% for excellent service for the meal, then you would leave 20% of the entire bill except tax (wine and drinks included.)
What if it is an expensive bottle?
Consider how you define ‘expensive.’ Living in Los Angeles, I would consider this to be anything over $125/bottle. For anything less than this, I would tip as normal. For wine over $125/bottle, if there was no additional or exceptional service, then I would be inclined to tip a smaller percentage on the wine (say 10%) then on the rest of the bill (20%.) I define excellent wine service as …