Steve is right on. I would recommend keeping builders no more than 20% of your business. You may have to wait 30-90 days for payment if ever and you will be doing the work at a discounted rate. Here is a great example.
A builder I have had as a customer for 3 years, good payer, reputable, and stable etc. I always throw in small extras like hauling unneeded dirt off the site for free, small things that add up but keep him happy also. He wanted a lot cleared that is 2.5 acres of heavily vegetated mature pines live oaks and pepper trees. I was going to clear it for $7k, very fair price right? WRONG!!! Some other moron bids it for $2k. WTF??? The guys shows up with a bobcat mini ex and a 5 yard dump truck. It takes him a week and probably $2k in dump fee's alone, not considering fuel. Basically he wanted to get in with the builder. Then on top of it he buys the builder an expensive bottle of cabernet to sweeten the deal even more. The poor guy never got any more business from the builder and we continue to do all the site work. Moral of the story is this. Builders have limited if any loyalty. You will become their go to guy, they will expect small extras, and count of financing the work also. All this at a discounted rate to boot. Is it really safe? No. But it can become good filler work for you and your crew. Ask the other subs on the job how they like working for them and how long they have been working for them. You're better off staying away from bigger companies like Lennar, or Mercedes. Stick with a smaller localized builder who does high-end custom homes. Spec house builders are a corporation without a face and you will have very little to any knowledge how financially stable they really are. Hope this helps you out.