An Introduction To 3D with Lyndon Daniels
Part 3: Polygon Modelling Tools
iTry utalizing your 3D aplication's Snap to grids, objects or components options as this will allow you to model with more accuracy around the mirrored plane.
Snapping to grids is especially useful for modeling your character's head, as when we decided to model half of the head we also chose to create an imaginary plane running centrally through the character's head, flush with the origin and parallel to the YZ plane (asuming your applications Y axis points up and Z, forward).
Having vertices that cross over this plane is not advisable, because they are crossing over into the opposite half of the character's head space which is reserved exclusively for mirrored geometry, potentially resulting in overlapping edges and unpredictable deformation in the event that the head is utalized for animation.
Snapping all the vertices that comprise the head's mirrored plane to the imaginary YZ plane, will also ensure that when it comes to merging the two halves of the head that the two halves lineup exactly (edge for edge and vertex for vertex) which makes the process of making the head a single continuous mesh that much easier.
The process of snapping vertices to a plane will often involve using your 3D application's snapping tools in combination with the standard transform tools. However, some applications have specific tools to prevent vertices crossing over a mirror plane, such as the ability to clip the movement of a vertex when it intersects the mirror plane or merging vertices automatically when they intersect the mirror plane.
In the previous image the vertices displayed in yellow have been snapped to the YZ plane, which marks the axis along which our model is mirrored.