If using the center line to Snap To for initial alignment of the two halves, you can get greater accuracy in the Snap To function by magnifying the view to 1600%. The Snap To accuracy is relative to the viewing percentage, and it can vary quite a bit; it may affect the end result if you aren't very careful.
If changing the position of anchor points on either half of the shape, be sure that all points on the half are selected. If you don't select all the points, the "perfect" halves will distort in accordance with the size of the move. Using the Component Selection tool exclusively during alignment can keep you from making selection mistakes. If single points get moved, it will be best either to revert by using Undo or reduplicate the companion half.
It is important to understand the different results that will be attained by choosing the different modes on the Options bar for combining for the paths. Different choices may yield very different results. The options on the Options bar are (from left to right) Add to Shape Area (Combine All), Subtract from Shape Area, Intersect Shape Areas, and Exclude Overlapping Shape Areas. Your selections and the order of selections for each path being combined will affect the results.
Manage custom shape sets for convenience, and be sure to save them after making additions. If you fail to save a shape set, if you reset or replace the shapes, or if you lose your preferences without saving the set, you can lose the shape(s). Saving the shapes in dedicated sets is also a good idea because you won't have so many to wade through if you're looking for something in particular. Manage what is available using the Presets Manager or the menu on the Shape Picker (accessed through the Options bar with the Shapes tool selected).
Envision the end result before you start creating paths. If you know what the whole thing looks like, you will be better able to sketch half of it with the Pen tool. The more adjustments you need to make by re-creating and realigning, the more significant the amount of time you'll have to put in to create the shape.