While it is encouraged and even optimal to have the Res before the Pump, it's not 100% doable in some aspects. It really doesn't matter where the Res is so long as it's supplying coolant in the loop. The biggest issue of note is out of the Res to Pump order, the loop will take longer to fill and rid itself of all the little bubbles. So if out of order, make sure to tilt the case to the angle that will accommodate keeping the pump lubricated. This is 100% the key to pump operation and health. Run pumps dry at your own peril and expense.
Some instances where it's not doable? I have one. In a mITX or SFF build where space isn't at a premium. But so long as the Res is always higher than the pump it should be a non issue. Another is that some builders can only put their Res alongside the I/O slots in their case above their PSU where pumps are not easily mounted.
I would recommend however that it be set up pump after Res whenever possible.