Doing a manual rollout calibration is the only way you'll really know the computers are set up properly. There are too many variables that can change the actual circumference of a wheel -- brand and tread of the tire, width of the rim, weight of the rider and the bike, inflation pressure -- for the default numbers to be anything but an approximation.

When you do the rollout, be sure to do it with the tires properly inflated and with the rider's weight fully on the bike. Do it at least three times and average the results.

Finally, keep in mind that different computers use different sampling rates; they don't update the "real-time" speed readout every revolution. So there could always be moment-to-moment differences between two different makes/models. A computer like the Panoram, which devotes more "cpu time" to displaying a bunch of data all at once, might conceivably reclaim that processing power by doing its averaging less frequently. (This is just speculation.)

IMO, the best way to gauge a cyclocomp's accuracy is by riding some measured miles. If the odometer function is accurate to within a few percent, the rest will take care of itself.

RichC