Keith, the dynamics of the trailing arm bushings are very different from those of a front control arm and even to that of the panhard bar. Rubber bushings twist and allow for vertical movement of the arm, as well as torsional twisting when one tire moves up and the other stays down (as in cornering or when a bump in encountered, and the axle doesn't move vertically in a perfectly flat plane). With the urethane, the inner sleeve pivoting within the bushing will work perfectly if the axle moves perfectly vertically at both ends simultaneously. But this seldom happens, and most of the time a twisting motion is encountered. But the urethane is MUCH harder and resistant to this plane of deflection, so it benefits us to put as much compliancy (in the way of rotation) into the urethane as possible.
If the harder urethane does deflect during a twisting/lateral motion (it does), isn't it going to make it more difficult for the sleeve to rotate within a distorted bushing? Lubricating the OD gives another avenue for the rotation to occur in. So I lubricate the ID of the trailing arm, the OD of the bushing, the ID of the bushing, the OD of the metal sleeve, and the ends of the bushings where they contact the chassis mounting tabs. The benefits of this can be verified by installing the bushings with only the ID greased first, installing the rear axle (sans springs or shocks or panhard bar), then with a jack under the center of the diff, turn and twist the axle along the centerline of the T/T. Now try the same experiment with a fully greased bushing. In fact, try it with OEM rubber bushings...they actually have their own 'spring rate' and will pull the axle to a neutral position when released. But compliancy is better for ride and performance.
The life of the bushings will be extended by lubricating the OD of the bushings as well, I have proven this over the years from racing Opels. I can put 5 years worth of wear into a bushing in just 3 or 4 weekends of racing (not an exaggeration!). In fact, I tested Gil's (OGTS) prototype urethane Manta/Ascona bushings in my street car many years back, and wore them out in 8 months of street driving. So this gave Gil a good idea of some potential problems after I removed them and diagnosed the causes, and he had the bushings retooled so this would not affect his customers. I have installed probably 40-50 sets of rear poly bushings, and am simply conveying my opinions so that others do not encounter problems I have had in the past. Besides, the instructions Gil provides with the rear bushings specifically state to lubricate all surfaces, including the trailing arm ID and bushing OD.....
In fact, my prefered methodology for trailing arm attachment is utilising urethane bushes at the trailing arms where the attach to the chassis, and spherical rod ends at the attachment to the rear axle. Zero slop, but less binding than all urethane, and better chassis noise/vibration isolation than all spherical bearings, with better lateral location as well. Hope this all shows some insight into my thinking.