In general more space between you and the airbag is safer. You want it to be fully inflated before you hit it. But they're designed to inflate in the time it takes a typical person to impact the airbag. Basically all you need to do is sit back in your seat and in a comfortable position and you should be okay.
Every now and then you see nervous drivers sitting forwards in their seat with their face near the wheel. It's scary to think what would happen if the airbag were to deploy. Just don't be that person.
Also, be careful sitting too far away that you're not locking your elbows. You might cause serious injury to your arms in even a minor crash.