It's a good thing to insert vertices multiple times without moving any of them. The shape of the model stays unchanged and the opposing side vertices are being placed exactly right, meaning that the top matches the bottom and the left matches the right. Sometimes I add a lot of vertices (but always an equal number of vertices for symmetry) until I get the ones I need. Then I delete all but the outermost ones.
This way I ensure that the ones I choose to use are perfectly aligned. For geometrical and symmetrical shapes it's always a good idea to avoid moving any vertex or surface manually by dragging it since 1) perfect symmetry will be lost and 2) it may inadvertently move the vertex along an unintended axis at the same time, especially in 3D view. However, if we set the positions by entering exact coordinates this can be avoided. I like the idea of "resizing" a symmetrical selection of vertices--I've never thought of that option
