Yes it is, but you're obviously wasting the capabilities of the triggers just using dumb manual flashes.
ETTL triggers aren't just great for using ETTL as an auto mode, you can manage the power of the flash guns in manual from the camera position too.I find that by using manual flashes has helped me to understand what each light is doing and how to control it. Although, I'm relatively new to off camera lighting and I suppose that once I've gained more experience and confidence i would be happy using other techniques too.
We always use manual in a static situation said:That, I didn't know. Not having to run around changing power outputs would be so much easier!!!!