Do you add exercises used to your MFP?

cafeconleche

First, let me say that I do not use a HRM, but I am looking to buy one in the future to track calories burned, etc. In the beginning of using this app, I would add calories burned to my MFP, and it would subtract calories for the day. But then I got to thinking, how does it know what I actually burned? Isn't it impossible to know, without closer monitoring (for instance, via a HRM, body bugg, etc)? So for now, I just use MFP as a food calorie tracker, which is amazing, but I am wondering if I am alone in this reasoning. It just seems that by adding the exercises without truly knowing how many calories I've burned would, in turn, make me go over my calories for the day? Do my rumblings make sense?:)
Also, for those of you that use HRMs, is there a way to add your calories burned without typing in an exercise? Just hoping to get the most out of this wonderful tracking app.