I would say most pros, if not all, swing to the right due to pronation, regardless of flat, slice or kick serve. In the following clip, Suzuki demonstrates all three serves (from left to right, flat, slice, topspin). The "entries" are different but the follow-throughs are similar.
However, they may or may not follow through to the left after that, based on personal preference. E.g in post #3 above, the server's racquet is on the right in frame 9, just like Sampras in post #2. However, in frame 10, his follow-through continues to the left. Would you say his swing ends on the right or left? Maybe the coach was just not used to seeing the swing end in a position like in frame 9?