Either way works -- cut a corner or make a small slit or pinhole. If you heat the shrink bag after making the slit/hole, you may find the hole will open up more as the heat causes the plastic to shrink away from the edges of the slit/hole. That's not a bad thing if you sell. People really want to smell the scent, and there has to be some kind of opening, however small, for fragrance to reach people's noses. If you're wrapping the bars for storage and personal use, then this point is moot.
What do you think of the smell through polyolefin shrink wraps?
Would you still make a sniff hole?