Franky, I don't intervene at all, but I am not raising pets. I know there will be tons of success stories, but generally speaking, those that have to be helped often do not do well. I want the best stock to survive, so I don't assist those that can't make it on their own. I realize that the reason some don't make it on their own may have less to do with the vitality of the chick and more to do with human error, but that's my policy.
For those who wish to intervene, I recommend that you not intervene before at least 24 hours have past since pipping. It is common for them to pip and then do nothing for a day. As long as your temp and humidity are good, let nature take its course. If you open the incubator, your humidity will drop instantly and any eggs that have pipped will be in danger shrink wrapping the chick because of the rapidly drying membrane.
If you must open the bator and it is portable, open it in a warm steamy bathroom after you have run the shower for a few minutes to fog it up, and then be quick about. I have done this successfully when I have had considerable differences (24 hours or so) between the first and last ones out of the shell and have to the get early hatchers to the brooder.
Also, TwoCrows makes a good point. A chick that pips is not necessarily ready to hatch. This is especially true of your earliest pips. At that point, the chick is still connected to the membrane and may be absorbing the yolk sac. Assisting at this stage will kill the chick. I find that the earlier the pip, the longer the wait, while the later pippers tend to pip and zip in close succession. It all has to do with when the chick is ready to hatch, which is not always indicated by pipping.
UGCM