Not a wine drinker, but it seems silly to me that someone bringing you a 100 dollar bottle of wine deserves an extra $14.40 dollars more in tips than someone who brought you a 20 dollar bottle. It required no additional effort and probably no additional skill.
I get the principle of percentage tipping because expensive food genreally requires an extra level of care and skill to make/serve. But unless the waiter went out of his way to suggest the perfect wine that turned your meal into an absolute home run, i don't think he deserves any extra money. And even if he does that, I don't think he should get a flat 18 percent on the price difference
How much more skill/care goes into
carrying a $50 steak to your table instead of a $20 pasta dinner?
Well is the pasta hand made? Is the steak cooked to perfection? Both can be tricky. Also, the cost of the steak is much higher, assuming you are getting a quality steak for $50, making the margin on the pasta much better unless, it is hand made where you must account for the cost of staff. Too many variables here to determine an answer. Please add additional color.