Core-punching is one of those subjective topics. It's inherently dangerous from the outset, so rookies or newbies I would completely discourage. However, with a level of experience comes the possibility of successful core punches, and with much experience this possibility often becomes standard practice.
To start, it's obvious that if you've managed to position yourself favorably in relation to the storm, there is no need for a core punch. That's the easy part. Now, the reality is that even the most experienced chasers aren't always gonna get it exactly right on approach or forecast, so the inevitable need for a core punch will be in the cards.
The most dangerous part about core punching is not understanding storm structure; as I mentioned above, I completely discourage rookies and newbies from trying this. Even though the core punch is dangerous by nature, it can be done successfully with an understanding of storm structure and an old-fashioned sense of survival instinct; i.e., knowing when something's not right before you see it happening. When you know what you're up against, you've won half the battle, because knowing the structure also means you can judge the exact amount of danger posed by things like your distance from the updraft as you begin your approach, the direction of storm movement, the speed of storm movement, and the type of storm you're dealing with. When you have good experience in understanding and deciding upon all these factors, the level of danger from a core punch is cut in half.
Using the experience example above, there are still storms that even the saltiest of veterans will not punch, in particular a nasty HP with strong rotation. Then there are other situations where a chaser will choose to core punch, even though it may be dangerous in itself, because they have (and use) the experience necessary to pull it off. You can penetrate a core and come out into the clear directly in the path of a tornado, but when you're expecting it, it's not a shock or something that would cause panic. The chaser realizes the high-danger factor all throughout the core punch, and despite being in a tight situation (relative to the tornado), they can methodically move through the core, into the cage, and successfully observe a tornado from close range, without becoming debris.
One of the most dangerous aspects of core-punching (after lack of structural knowledge) is the panic that can be induced by not knowing or expecting what you might run into. Tight situations call for quick (and good) decision-making. Lack of knowledge schematically and lack of actual field experience can seriously hamper a chaser's ability to do this. So to wrap this up, read up on structure (the different types of supercells have different structures and personalities), go out and chase and get some experience around storms, and then after a few years or so, when you're brave or confident enough, then you can try a core punch. But bravado alone will get you in trouble; you need the seasoning.
It's tough sometimes to purposely back off when you're a rookie or new chaser, especially when you haven't seen much yet and you know the biggest prize of your career might be lurking in that core just a few miles away. I was faced with this decision twice during my first season and both times knew there was a confirmed large tornado on the ground inside the core. It was a bitter pill to swallow knowing what I was so close to but yet was still missing, but both times I didn't feel comfortable punching, so I backed off. The first time, the tornado dissipated about a mile southwest of me (I could never see it, found out through LSRs the next week). However the second time, on I-40 west of Lela, TX on June 11, 1997, we found a 3/4 mile-wide damage path less than two miles ahead of where we'd stopped.
