Have you tried using a fetascope? We used a doppler throughout my second pregnancy, before I found out that the sound waves are actually more intense than with an ultrasound.
We didn't check heart tones. I think though that if there was anything I was going to do, in terms of monitoring things, that would be it. Basically what you want to listen for is anything drastically out of the ordinary; obviously a very slow or very fast heartbeat is going to indicate stress. It is normal for the baby's heart rate to drop a bit during contractions at the end, I believe, but it should come back up in between. And it likely also that you'll lose the heart beat entirely as the baby is descending throught the birth canal -- it doesn't mean the baby's heart has stopped beating, but to not be able to find it can be stressful and take the focus away from where it should be for the birth to proceed normally.
ETA: FWIW (and honestly I don't know how much it's worth
) Rahima Baldwin says 110 or 160 is cause for concern and further monitoring, and below 100 or above 180 is serious. Here also is a bunch of midwifery info about FHTs:http://www.gentlebirth.org/archives/decels.html