Whether to clamp before removing your syringe, or clamp after the syringe is removed really depends on what type of injection cap is at the end of your IV catheter.
There are NOT caps which are "positive pressure", but rather the caps are: Positive Displacement design, Negative displacement, and Neutral devices.
The Positive Displacement caps like the CLC2000 and the Maxclear, are devices that should be used in this fashion: Flush the device, leaving approx. 1 cc of NS in the syringe and then disconnect your syringe, wait 5 seconds then clamp.
The reason for this procedure is that once you remove your syringe, fluid is forced out the end of the IV catheter which then disallows a blood reflux into the catheter.
The Negative displacement design caps should be used using positive pressure flushing: Flush, and then as you are flushing the last 1/2 cc into the IV catheter, you clamp the IV catheter simultaneously.
Or, if you have a neutral design cap, the flushing sequence is inconsequential.
Hope this helps. Check out the IFU's on each caps web page to assess its type.