I was typing ,but SDave answered first... so its pretty similar.
It depends upon what style of recurve you make. A static recurve, none of the recurve appears to bend, because its left stiff on purpose. Active recurve, the recurve will appear to bend, straighten out a little, but not fully. Static has no set in the recurve, whilst active recurve will take a little bit of set, but still look recurved.
Most straight bows do well with 4"- 12" of non bending tips, depending on the style of the bow. If you tiller the bow whilst its strung, the string effectively stops you from working too close to the tips.
You know you are working too close to the tips because the string will get cut.
A Bowyer's Edge type scraper is a great tool for tillering. You just need to wrap the opposite side of the blade that sticks up with electrical tape, that will stop it from accidentally cutting the string.