I think you really have to know your incubator, your incubator humidity level, your ambient humidity level and how fast your incubator humidity recovers.
If you are having trouble keeping your humidity up in a cheap, unregulated incubator in an area with really low humidity, I believe that opening the bator while chicks are pipped can definitely lead to sticky chicks - especially for those people who have to check every egg, all the time, and don't even know their true humidity level.
Therefore, if you are not sure I think it is way better to leave the incubator closed.