How to know what labs for chemo?

I am starting on a chemo unit and I'm really confused on what labs are important for the various types of chemos. There are some that i have identified that go with certain ones but honestly, don't always understand the rationale behind it. I use the ONS chemo book and for many chemo it identifies certain labs but this is not true for every single one i give. generally i look at the cbc and chemistry and make sure everything is okay, but what throws me off is when other nurses start infusion when only the cbc is back, or only the chemistry is back, vice versa. When I ask them they say "oh we don't need the cbc on this drug", or we don't hold for chemistries on this one". Can someone help me understand why some things are important and others aren't, or if there are guidelines i should follow?