Do you have to use the heart monitor to get an accurate read during a workout

Just got my Apple Watch Sport. I'm using it during outdoor walking and indoor workouts. When I was doing my usual workout, the Workout app was showing my HR at like 60-70 when it was definitely higher than that. And it was showing minimal calories being burned during a pretty powerful workout. I had removed the HR monitor 'glance' from the watch, but decided to put it back. Once I did, the HR in the Workout app was way more accurate. The problem is, the HR 'glance' drains the battery really, really quickly if left on. And if it's in 'glances' it seems to always be on.

So, does the HR monitor constantly have to be on to get a good reading in Workout and Activity?

And, can it be turned off somehow without having to take it in and out of 'glances'?

Any help is greatly appreciated.