Most children establish a good pencil grip by the age of 4, however, this is not a hard and fast rule. A poor grip is often a sign of immaturity and will usually come right in time.
There are a few cases where poor pencil grip is a sign of further problems, for instance low upper body muscle tone, which will require attention from an occupational therapist. The teacher should monitor the progression with the child and act where necessary.
The importance is quite obvious: children need to be strong in this area to sit at a desk for lengthy periods of time, in preparation for primary school. If a muscle tone problem is noticed, early intervention from a therapist will save the child a lot of frustration later on, where he may find himself unable to keep up with the pace of the rest of the class.