I have flown 3D for years and never used a cutoff, (nothing worse than your esc cutting off while 5' off the ground in a hover) but I stop flying as soon as a feel a drop in power.
In surface I try to use a cutoff when I can but when I haven't been able to I do a couple of things,
First I run my car for a set time that I know will not drain the battery fully, say 6 min, then I charge the battery back up to see how much I used. I then use those figures to calculate how long it should run on a charge and use a timer set a few minutes short of my figures to protect my pack.
Second I pay very close attention to any drop in power and pull off asap.
Like most said, use a cutoff if at all possible but you can run without one if you are carefull. Also if you run cheap batteries you will have many more problems running without a cutoff.