I don't think it makes much difference if the leader is taped on or attached with a hook and loop product. They do need to be installed 'relatively' square. But "precisely or exactly" are not words I would use to describe their installation. I have yet to find anything precise or square about a leader. This is one of those "it's not rocket science or close enough for government work" metaphorical instances.
Mine are taped on and when I finished installing and or reinstalling, because I use 10' Leaders on a 12' table 95% of the time, and install the 12' when I get in a large quilt top that needs more than 110" of leader...which is not very often. I do this so that I don't have to deal with leader coming in contact with the sew head when installing a quilt. The shorter leaders give me plenty of space to clean or change bobbins and allows me to lay the leader FLAT on the table to load the quilt. I use Red Snappers and Red E Edge Clamps exclusively. I can quickly install a back and have a non movable surface to press the Red Snappers in place. I float my tops so there are only two leaders, they are both "Take Up Leaders" one on the Take Up Roller and one on the Bottom or Belly Roller. The leader on the lower Stationary Roller has been removed.
I also"center" the quilt to the table, not the leader. The center of the leader can move left or right, depending on how it rolls up on the bar. It's never the same. Every now and then it might roll evenly.
More often it will roll up unevenly. Same roller bar, different roll.
For this reason I don't center a quilt to the leader. As pictured the center of this leader moved approximately 1/2". So for me I center to the table.
The ruler is a piece of tape. In this case the "6" mark is the center of the table when the 12' Leaders are installed and the 'heavier' line between 11 and 12 is the center when the 10' Leaders are installed. When I install the leaders I center the leaders width to the width of the bar as close as possible. So that basically the same distance on the left of the leader to the tables edge is the same as the right. After that I have no need to know where the center of the leader is.
Time to load the back...square. With the center of the back creased in I lay the Take Up Leader sleeve containing the Red Snapper tubing flat on the table and as straight as possible. I have chosen the most square edge of the backs width for the installation on the leader. In this case it was a selvaged edge.
I then lay the back on the leader and install the Red Snapper in place making sure that the back is as close to possible to being square. The right edge of the back and the left edge of the back are the same. 99% of the time I do this visually. This time I took photos to demonstrate.
Now comes one of, what I consider to be, the most crucial part of loading a back square. Rolling it up onto the Take Up Roller. If you get it rolled up square here the battle is won. The back will be square to the frame, the top will be square to the back and the quilt when longarming is done will be a square as humanly possible. My first indicator is how the back is laying on the table when I'm ready to install the bottom Red Snapper. When it looks like this I'm a happy longarmer. If the back is at an angle to the table, which we know is square, it comes off and I start over. I've worked on this system now for over 600 quilts so I don't start over very often.
Once I get the bottom of the back connected to the Belly Roller and I start to feed the back onto the roller I watch for signs to confirm "square."
Is the Red Snapper as it is rolled up onto the bar even across the width of the back.
If you have a patterned back this will be the first opportunity to check that as well. Is the pattern consistent with the Stationary Bar, or in line if you will.
This is something I check for all the while I'm rolling the quilt. I know my bar is straight, I know I loaded the back to be as square as possible and checking as I roll confirms that the back is loaded square.
If I'm fortunate enough to have a relatively straight edge on the left or right of the back I check to see that is rolled up square.
This is always nice to see because the leaders most certainly did not roll square.
Time to load the batting and the top. I lay out the top and visually look across the quilt to see if it is square. I will measure to confirm from the top Stationary Bar to the first seam. Hopefully the piecer has sewn a relatively straight seam. More often than not the seams are. It really stands out when the first left to right seam is off. This one was good.
I measure across the top, again from the top Stationary Bar to the first seam in 4 to 6 locations making sure the measurement is the same and one last look across the top to visually insure the top is square to the back.
Quilt till you wilt...we do.