is this faster/more convenient to use than other methods?
Well, seems a good way to me. I assume you do this at the beginning of the game, I mean, you first 'prepare' the terrain, and then show it.
If so, and if is fast enough, I would stay with it.
Anyway. If the terrain modifications are always the same, and does not change during the game, you could think to modify directly the heightmap accordingly, so later you do not need any other rounding.
You could modify the heightmap on the fly, plotting a lighter color where the terrain should be higher, directly on the heightmap file.
I would not bother though. Seems to me a valid method.
Sergio.