As stated above, Balsa is commonly a started wood for crewing. Pine is clearly a better long term choice. NOTE: Choose non-construction grade pine for your tiel. Construction grade is for MAC's, Too's and other larger parrots.
Tiels like to crew at the edge of expensive woods, as do most parrots. Providing a suitable and less costly pine is a great choice!
Personally I use Balsa because my parrots absolutely adore it, as does my rabbit. But I always try to make sure they aren't eating any and that it's only being destroyed. I haven't had a problem with it for the two years I've left them interact with it, and I'm hoping it stays that way.