ASTM E1554, RESNET Chapter 8, the Duct Blaster® manual and the Retrotec duct tester manual are all in agreement that supply registers and return grilles must be sealed during the duct leakage to outside test.  If you are not taping registers, you are going to have a hard time justifying it.

There has also been research that is in agreement with this requirement.  The Energy Center of Wisconsin did a study for Focus on Energy in August 2008.  The title of the study is A Field Study of Exterior Duct Leakage in New Wisconsin Homes.  Here is a link to that study:  http://www.ecw.org/publications/field-study-exterior-duct-leakage-new-wisconsin-homes.  Among its findings are the following:

“The reliability of measured leakage to the outside from duct pressurization tests depends on the level of leakage to the interior. High interior leakage makes for less precise measurements of exterior leakage.  This reinforces the need to properly (temporarily) seal registers before conducting leakage-to-outside test using duct pressurization.  It also indicates that exterior leakage measurements in homes that use building cavities or panned joists as duct runs are subject to substantial error.

Another aspect of the duct pressurization tests that was investigated was the impact of leakage to inside on the resolution of the estimates of leakage to outside.  Since leakage to outside estimates require that the pressure difference between the home and the ducts to be zero, and small error in adjusting this pressure difference could result in a large error in the leakage estimate if the leakage if the inside is large.  For example, it the pressure between the house and the ducts was actually 0.2 Pascals instead of 0 Pascals, and the leakage to the inside was large, the flow to the inside at 0.2 Pascals could be large, and would be read as additional leakage to the outside.”