It depends entirely on where on the bars you will be riding most of the time. If you are on the drops most of the time, then you will be reaching farther than you would on flat bars or risers. The same issue comes up when you switch from flat / risers to bullhorns. Since the difference in reach from flats / risers to drops will undoubtedly be more than 10mm, it stands to reason that you will require a greater change in stem length to maintain the same riding position. When I size a stem, I consider the totality of the setup, including bar reach and drop, bar width, bar rise / drop, hand orientation (palms down horizontal, palms in vertical etc). Personally, I don't like using track drops on the road, since they don't offer a comfortable position on the tops, and riding the drops for extended periods is tiring unless you set the bars up very high.