It is OK to Be a Demanding Customer
Having said how valuable and important a supplier can be to you, I'll now say that you should not be a patsy. You can be a demanding customer--just be fair. State your quality and time needs clearly. Hold your suppliers to their agreements. Make sure they stay competitive. Tell them you never expect to pay higher prices than other purchasers.
There are times you need to replace a supplier because you have outgrown them and they can't perform to your new expectations. Before dropping them, however, you might try to help them change to keep up with you.
Also, it's not prudent to rely on one supplier. If that supplier has a strike or a fire, you don't want to be in a position where you'd be shut down too. So keep a backup or multiple suppliers on hand--and don't be embarrassed to tell your key supplier that you're doing so. They will appreciate your honesty.