It depends on several conditions. The main requirements for the TPRV discharge pipe are listed at this link - http://checkthishouse.com/16/water-heater-tpr-safety-valve.html
However, your local jurisdiction requirements could prohibit exterior installation, if you experience freezing temperatures in your area, exterior TPRV discharge wouldn't be a good idea as weel.
Another issue is actual leakage of the valve which may not be noticable if you re-route this tubing to exterior. Leaking TPR valve may sometimes seal itself and stop responding as a safety device. In such case, without you even knowing about it, the valve could become a serious hazard.