You will get better acoustical performance with a gap (of course, more insulation is better, but the gap gets you close for less cost). I was going to go with custom ripped 2" frames (custom ripped because no material seems to be 2". Everything is pretty much the standard 2.5, 3.5, etc). Then I opted for using 1"x4" (.75x3.5" actual) aand it gives me a natural 1.5" gap, but the frame hides the air gap. I was afraid that my wife would freak out, but she really didn't. I first put the raw OC703 up on the walls by just hanging the OC703 with the hangers I had for the frames. I then put the OC703 into the frames and put those up. There WAS a noticeable improvement in the mid and lower frequencies. These aren't bass traps, but the clarity in the LFE effects was greatly improved. I wouldn't go back to a no-gap setup because of the improvement.
Now, if you can't do the gap, or it doesn't look right or gain household approval, then don't sweat it. You are missing out somewhat, but too many times people here at AVS make people feel foolish for not going "all out and doing it perfect the first time." You are putting up custom covered acoustical panels! THAT in itself is both cool and a very good sound improvement. Now if you were doing 1", then it would be a different story as 1" doesn't really do much of anything.