I'm not a breeder of parrots but I do keep reptiles, aspen is good because it has no strong smell and is virtually dust free, however it does mold fairly easily if it gets wet and must be changed often.
I have heard that aspen is one of the best materials for nesting cockatiels! But like the other commenter said- make sure to change it fairly regularly as cockatiels will often sit on their eggs while slightly wet to provide humidity (and this could cause mould/fungus if the bedding is not changed!). Another good option is wood shavings.