Yeah, the availability of T-nuts / threaded inserts is a pain --- the pricing in small lots excrutiatingly so.
If one is doing lots, then arguably one might as well do a table material which will allow for threads --- easy enough to drill out and install additional threads.
I did a central, radial arrangement using Phi when I did my threaded insert spoilboard: http://www.shapeoko.com/forum/viewtopic.php?f=35&t=6045 and it worked well.
Currently using the central T-track arrangement: http://www.shapeoko.com/forum/viewtopic.php?f=37&t=8332&p=65068#p65068 (yeah, I'm a big fan of symmetry).
For all that, I think an arrangement:
- starting from the corner origin
- spaced on phi
- with the first set spaced slightly larger than the smallest block of material one wishes to use and the spacing always being at least half clamp length of additional space