That is much better than doing it with annotations because it's less intrusive but I still don't use that feature.
If someone is watching my video, then I don't want to do anything that will distract him/her from watching as much of that video as possible and then hopefully watching another of my videos and then another. I tend to focus on the views and watch time and I let the sub count take care of itself. If people want to sub, they will do so. Also, it's perfectly fine to have a "sub" annotation in the outro at the end of the video if you want to remind them.
In the long term, it's views and watch time that will drive the growth of a channel, not the sub count. Some people sub to a channel as a way of "bookmarking" it or putting it in their "favourites" and then they go off and do something else without actually watching the end of the video or watching any videos at all and then they never return because they don't really know your content.
