When to take the first catcher on your board is usually a tough call. Like a top tight end in football, a top catcher gives you a huge leg up on the competition who might not be getting much from that slot. But because tight ends have to block, and catchers have to don the tools of ignorance the upside for players at those positions is limited, and the risk is often high.
I typically prefer to target two solid catchers - the Bengie Molina/A.J. Pierzynski types - later in the draft, though I have been known to reach for Joe Mauer on occasion because his batting average from that spot might be more valuable than that of any other player in the game (consider the average catcher batting average versus the average outfielder one. A catcher who can hit for average is like an NBA center who can hit 85 percent of his free throws.)
But by and large, you're better off spending early picks on other position players who will log more at-bats, are more reliable and have more upside. The position scarcity angle makes more sense in the middle infield where the demands of the position don't interfere with the player's ability to produce at the plate.