IIRC, tarantulas aren't so mindless in their eating that they put themselves in danger physically. There are some creatures that will literally eat until they explode, but I don't think you can overfeed T's... if they're hungry, they eat. If not, they won't. And they sometimes go months without eating with absolutely no ill effects.
Of course, I may be totally wrong, but to the best of my knowledge, T's will only eat what they can handle...
They won't eat to the point of explosion, but that doesn't mean you shouldn't exercise a little parental control. A particularly rotund specimen is more prone to abdominal ruptures from falls and formation of abdominal calluses (which often result in eventual spontaneous rupture). Since they evolved in a situation of not knowing when the next meal was coming, they will happily eat to their physical threshhold on a continual basis, so any control has to come from you. I don't follow any particular regimen with feeding, I just feed often enough to keep everyone in proportion but not HUGE.
I try to stay with nature more in terms of timing of feeds. In other words, I feed my T's whenever I feel like it, with limits ofcourse. I sometimes let them go over a week without food and sometimes I feed them 4 times a day.
I agree with codemonkey. We have to note that spiders hardly ever get as big in the wild as they do in captivity. A spider with a destended abdomen puts it'self at a risk for poping like a overfilled balloon if it were to fall on something. But this rarely becomes a concern in normal feeding regimnes.
Certain species are more prone to these problems than others. Typically, the slow metabalisims of genra such as Brachypelma cause them to be more likely to develop huge abdomens.
Here's a pic of my biggest B. albopilosum (courtesy of Art Evans). She ballooned up even bigger than this, but after dropping an eggsac (a year and a half after breeding!) she returned to normal perportions.