Although I am but a rookie, it is my opinion that it is worth the expense/weight to get one of those voltage alarms. After having battery issues, I tried one and will never do without one now. They alert by individual cell voltage. So, if you are working the battery near its C rating, the alarm will sound before single cells can drop into the danger area. This detail seems to be important, at least with the cheaper zippy/turnigy batteries that I use. I use the cheap ebay alarms and they work fine, but they are reasonably priced in the FT store as well.
P.S. If you do go the alarm route, get two or three of them. They disappear easily in the grass when/if you crash.