>Use anything white at the site. There is always something. I >use napkins, a white tee shirt, a table cloth, anything white. > Shoot a WB preset of the item then shoot away using that >preset.
For the most part I would agree with you, except for one point.
Using anything white at hand is OK as long as OBs (optical brighteners) are not present.
Some soap manufactures use OBs to make your whites look whiter; and some paper manufactures use OBs to make cheaper pulp that would normally look yellowish look white, and some white papers with a yellow tinge look even brighter.
OBs will give you an errant pre-WB setting. If you want something cheap thats easy to carry around and give relatively good results, white coffee filters work well.
Actually if you wanted a good read on a comparison of different WB setting products, check out this article. The surprise of the group was a Melitta white coffee filter for $.01 compared to much more expensive commercially produced products.