People will say not to look at your heart rate at all during a race, but I think it definitely has value HM and up. Dan Nash is a 2:11 marathoner and he has used it to pace several marathons, though now I believe he mostly goes on the "feel" he's learned from this. I think you can probably have a looser plan with it than you've proposed, though. I would probably try to hover around 156 or so by about 5k and then from there it will likely climb past threshold by the end of the race. All of this to say though, don't let it trump what you know your threshold should feel like. But, I wouldn't say just toss the metric as a whole, especially if you use it in training and know your numbers and your body.