DFM checking is a necessity?
DFM is short for Design For Manufacturability.
Many electronics engineers don't have a good knowledge about printed circuit(PCB) fabrication technique. After the layout job done, the PCB gerber files should be sent to the PCB Manufacturing Engineers for DFM checking to avoid the possible risks and optimize the fabrication process（higher qualification ratio and reducing the cost). A DFM report includes the trace width/spacing、drill diameters、annular rings etc. A successful product is the joint efforts from every process. And the report is a important feedback to the electronics engineers to achieve the best blance between functional and manufacturability.
It is quite common and practial when talking about a big project or serious business.While it is quite often many PCB gerbers are not DFM checked in the so called fast prototype service provider. It is of high risk. And factories of this kind would just fabricate the boards like "what you see is what you get". Any problem for the PCB gerber files would ruin the boards and the fast prototype service provider would not be responsible for that.
So the best strategy for that is to ask before reaching a deal. DFM checking is like a safty belt, you won't see until there is a bump on the road.