I tend to look at the cards when I sort them out, but only at a glance, as I'm more focused on sorting than studying the cards. So when I encounter them later, I still have to look at them to see exactly what they do.
I do my best to keep myself as unspoiled as possible. I like turning over a card and seeing its effects for the first time, especially with villains and henchmen, where you sometimes have to go into panic mode and be like "oh, well I didn't know he did THAT." Especially if, say, the villain wants or does not want two characters at the same location. So I try to glance at cards out of the corner of my eye as I'm cracking open the packs and shuffling them into the decks, and I try not to look too hard when I'm rooting through the types of boons for a specific card.
Besides, not knowing what awesome stuff you're missing out on means there's less to be disappointed in what you don't get... until you hit the end of AP4 and happen to glance the Navigator's Musket from AP3 hiding among the weapons. :(
You aren't the only one. My group agrees with me on not reading any cards until you actually encounter them. With the exception of making sure all the cards in the Cards List matches what I got in my package.
If we closed a location and there are leftover boons/banes we have never encountered before, we do not read them, even as I put them back in the box, I just sort them back into the box without paying much attention to its powers/difficulty. This includes henchmen and villains. We do not know how to defeat them until we actually encounter them first.
This has been fun for us for example with Mammy Graul. We were down to our last couple of turns and we thought we had the villain cornered before we encountered her for the first time. When we finally encountered her on our last turn, we did not expect her power and thus we failed the scenario.
I think it makes sense when you do not know what's up ahead and it creates more suspense and excitement. That makes the experience better in my opinion. Obviously it is different on your 2nd, 3rd, or 4th run of the adventure path.
That is why, albeit going trough the same AP multiple times with different set of characters is fun, I am glad Paizo keeps dishing out new content and have plans for new content for a couple more years.
I never read the card before I encounter them... Except when there is already a debate on a specific card on these forums and I have a chance of a enlightning exchange with Hawk, Orbis, Vic, Tanis and the not-this-what's-his-name-again-Mike-guy.