Standing seam roofing has been around for about 1500 years - so has this misconception: you must use tapered legs to compensate for expansion of material. Though expansion is an important factor in the installation of long panels it is not a concern when it comes to the width of the panel.
Take for example zinc, the material most susceptible to thermal expansion. If a ten foot long panel is installed on a day where the temperature on the roof is 40 degrees Farenheit, then on a summer day when it is 140 degrees Farenheit the panel will expand .209". That's less than a quarter of an inch of linear expansion for a ten foot panel!
Now consider the panel's width. When finished the average panel is between 12" and 25" wide. So if we say our ten-foot panel is, for instance 20" wide, then we can expect to see .0348" of expansion over the panel's width. This is just over 1/32" expansion - if you installed it at 40 degree temperature!
Since most roofs are installed at a higher temperatue or even in the extreme heat of the summer the panel is already expanded and can only shrink when the temperature decreases. Tapered seams can cause problems when power seaming. It can lead to both boxed seams and/or the seamer coming up off of the seam.