I like it after since any restriction in the pump suction has the potential to produce problems with fuel flow. For the same reason, you don't want to use any 90 deg fittings or other sources of restriction in the feed line to the pump. Also, the larger the line sizes the better.

The downside of putting it after the pump is the pressure build-up if you forget to open it and the potential for a line to burst and spray fuel everywhere.

PaulD