Since I am new to all this stuff and really have no idea how my body responds yet, I am getting blood work done very frequently. Right now I am in the paranoid state and doing them every two weeks. Sure, it ends up being a little over $100 a month, but considering how much money some people spend on gear, it is a drop in the bucket...and I get to learn how my body works. It also allows me to fix problems before they go too far.
For example, I recently found out that taking joint supplements (standard glucosamine stuff) caused my fasting blood sugar level to rise by almost 30 points over the course of about 6 weeks. Yeah, I stopped taking them. Turns out my entire family (except my dad) became diabetic in their late 30s or early 40s. So some genetic trigger is tripped and suddenly the chance of becoming diabetic goes from rare to probable. That, of course, is with on your normal American diet. I am on a 3J diet and have almost followed it to a T. That is why I was so shocked when I went from the 80s to 110 in a scant 6 weeks.
My point? The price of blood work is worth it...just do it.