The Dynkin diagrams of type ADE are ubiquitous in mathematics as solutions of various classification problems. The diagram E_{6} is usually drawn by five dots in a row with a sixth dot above the third, see for example here. There would be many other ways to draw the diagram E_{6}, for example the sixth dot below the five dots, or just a capital

E.

Is there a reason for drawing that diagram in that particular shape beside or is that just a confirmed habit?

thinkJohn Baez draws all his Dynkin diagrams such that the vertices are arranged in order of the dimension of the corresponding quotient G/P by a maximal parabolic. – Qiaochu Yuan Apr 9 '10 at 16:21