I agree with Bob's and Jude's advice. Sometimes, it's a "moment of contemplation" of whether you should go into the stack, or not. Just take a minute, and look things over. If you're going to push the balls into more balls (and create more clusters than you had before), you need to carefully weigh the benefit of going into the stack from that angle.
I never knew this until recently, but I'm more apt to go into the stack from underneath (having a security or insurance ball by one of the corner pockets) than from any other angle. After one day of noticing this, and asking myself the question why, I guess the answer is because I'm pushing balls into more open space, than forcing them into a nearby rail or congested space.
I also happen to prefer behind-the-rack breakshots, for the same reason. The experience dealt to me with this type of breakshot may have an influence on why I'm more apt to go into a semi-broken stack from underneath, than from the side or up-top.
Also, when going into the stack, I try to pay careful attention as to what ball (and what "side" of the ball) I hit. It might be a zig-zag four-ball pattern that, if hit on one side of the ball, opens them up like a flower; but if hit on the other side, sends the cue ball dangerously into another cluster or towards a pocket. It really depends.
Taking Bob's/Jude's advice as a general wiring harness or guide is wise, though.
-Sean