You don't need to treat, ever. If you are treating, you should make sure there is a grit source available (really, any time they aren't eating crumble or pelleted feed). Many people overdo treats - because the bird seem to love it, and it brings them pleasure too.
I'm going to caution against that.
Nutritionally, the most important time for your birds is their primary growth stage - first 12 weeks, at least. That's the period when the very best thing you can do is skip the treats, spend on high nutrition feed instead. Once their bodies are mostly put together, if you want to skimp a bit on feed to make budget for a tiny amount of treats, it will have limited impact on their further development and long term health.