That's probably the most common offset that guys use with 18x7.5" wheels on 6th gen Accords (probably because it seems to be the most common offset in that size and lug pattern). So the 42 should be fine. I'd expect some small amount of rubbing in the rear. It may require trimming of the plastic tab on the rear bumper, or rolling of the fenders if you camber is corrected back to stock specs. With uncorrected camber, a prokit drop, and 42 offset wheels with 225/40...I'd say you'll probably only rub with people in the back, or under hard cornering, and most of that will be alleviated by trimming the bumper tab.

I have 45 offset in 18x7.5 with 225/40 tires. With uncorrected slight neg camber in the rear, I never rubbed for two years, with people, luggage, never rubbed. So when I got another new set of 18x7.5 I made sure I got 45 again, cause my friends with 42 had some slight rubbing. Now I've corrected my camber, and it only occasionally rubs if I have people in the back or under hard cornering.

I'm rambling...bottom line...you should be fine. Get 45 if you can, but if you can't find any in 45 that you like, then get the 42's.