Do be careful. Becoming overly dependent on upstream "feeders" is what killed my company.

It is a balance. Turning down work is dangerous, and the generally opposite of why you're in business. But giving other firms too much leverage will kill you just as dead as insufficient work. I started getting nervous when one company was over 50% of our income, but there was disagreement about urgency and what exactly to do about it with my partners, and we dithered until the bomb went off.