My temptation with a heart rate watch is to stop the tracking immediately after I stop the activity and then do my stretching / cool-down. The problem with this approach is that the data around how quickly the heart rate comes back down during the cooldown is useful data.
It's obvious looking at a longitudinal heart rate graph where the cool-down is, so including the cool-down in the data doesn't really meaningfully pollute the data in any way. Also, my cool downs tend to be of a fixed 5 or 10 minute duration.
The only problem with doing this is the stretching time is included in the volume calculation for the week. Honestly, if getting some credit for actually stretching motivates me to stretch, I'm OK with the volume number being inflated by 5-10 minutes per session. Again, for any individual workout, it's obvious where the work was done, and where I was cooling down.
Another way to look at this data is in a weekly or monthly report from Polar Flow or your tracker's software. Here I can see the number of minutes total in various heart rate zones. Looking at the total minutes in the green 70-80% zone, and the blue 60-70% zone gives me a good idea of where I am, and where I want to be for the week. Looking at these numbers tends to leave out the stretching, even if I don't shut off the watch immediately after the actual work is done.
