One thing that always bothered me with the newest version of Omnigraffle for Mac was that every time I would drag a stencil to the canvas the toolbar would change. This makes it tedious to create a larger topology. Turns out there is a very simple way to handle this. As seen in the animated GIF below, all you need to do is double click on the stencils item in the inspector and it locks that particular inspector. At this point you can drag as many stencils as needed to the canvas without it changing the inspector on you.
I can’t take any credit for this particular tip. I shot an email to omnigraffle support expressing my frustration and they quickly replied back with the solution. Thanks OmniGroup!