Hi hming,
Glad to learn you're using QuickBooks Desktop to help streamline your engineering consulting firm. I can see the importance of being able to set different rates for your clients and I'll be happy to share more information on this so you can focus on what you do best.
Since you're using QuickBooks Desktop Pro, you'll have to manually set the price for your clients each time you create a sales form. However, with QuickBooks Enterprise, you can add the new subscription-based feature called Advanced Pricing. Advanced Pricing allows you to automatically change the prices of items on sales forms according to the rules you create. I encourage you to learn more about this feature using this link here. The article goes more in-depth on how this feature works and other alternatives you can take advantage of.
Let me know if this info helps. In case you have other questions, feel free to reach back to me in the comments below. I'll be one message away. :)