Mine are nearly identical to those described in Jack Sobon's "Build a Classic Timber Framed House". 5x7 rafters tapered to 5x5 at the peak (reduced to 4.5" fork and tongue joinery) set 32" OC (the two center most pairs are set with a 32" gap between them to allow chimney clearence). The rafter's in Jack's book span 20ft on a 9:12 pitch, mine are pitched 10.5:12 so they are just over 13ft. in length.
These proportions look really good and the 32" OC leaves a reasonable span for sheathing to bridge. 36" would make my rafters look (and possibly be) a bit undersized, 6x8 or 9s though impressive might look too fat for their overall length and too large for the space. So I'd say spacing in part depends on span
I agree, 24" OC works for purlins broken by principle rafters but starts to look crowded in long rows of common rafters.
