It doesn't matter as long as it's positioned before any device that could decrease flow while the pump is running. I.E. plugged filter, closed shutoff, or faulty sprinkler valve. My installs go...pump-spigot-relief valve-filter-shutoff valve. Spigot goes on top (same as yours) to make priming easier and before shutoff in case one wanted to use a garden hose off the spigot, they can close the shutoff and still be protected with the relief valve.
As for the relief valve...not a good idea. That relief valve is preset at 30psi. Most sprinkler systems run closer to 40psi operating pressure. So that valve would blow before your sprinklers even popped up. The adjustable relief valves I linked are more expensive because they are adjustable. I would look for one that is preset closer to 60-70psi. But, you have to be careful, because your system may be operating in this pressure range. Of course, the adjustable relief valve takes all the guess work out of it!
This post has been edited 1 times, last edit by "Central Irrigation" (Jun 28th 2012, 8:16pm)