hmmmmm ... i've only tried once to cut one long strip for a border. so of course i picked something with a slippery finish on top.
one thing i did some time ago was track down and purchase a 36" rotary ruler. i needed a 9 foot stretch, so i still had to fold the fabric. i put my mat on the floor and used my knee to hold the ruler in place. i probably had an elbow or two but i tend to not worry about such things unless the ruler slips waaaaay out of wack. i lay the strip down, line up the edges that are straight, pin it all in place, then ignore the elbow and focus on the straight line formed by the other piece of fabric or the top.
the long rip is certainly a sensible way to go, but i'd do a test rip first in case the fabric isn't woven evenly enough. clip about a half inch inside the selvage and rip down the length. measure to verify that your ripped strip is the same width all the way down. if it is ... go for it. if it isn't, clip and rip another inch to see if that comes out even. if it doesn't, you'll have to ignore the grain completely when you cut.
if you have a tile floor you can also do what i do when it's time to square up a top. run a straight line of string, held down at the ends with masking tape. line the first edge of your length of fabric up under the string. tape it into place with little bitty pieces in just a few places. measure and cut down the length in sections, moving the mat and keeping the ruler straight as you go.
(for a top i run all four strings using a carpenter's squaring ruler to form a frame the right size. then i move the top around underneath until it's centered the way i want it and trip carefully along the string.)
there's also the method used to mark straight lines on walls with a chalked, weighted string. tape a piece of weighted string to the wall near the ceiling. the weight will cause the string to hang straight. once the string has stopped moving, pull it taut and carefully tape it in place at the bottom. pull the string in the middle and let it snap back against the wall. you'll have a nice straight line. if you've lined up and taped your fabric to the wall under the string, you'll end up with a guideline to reference when you lay the fabric flat for cutting. (unless you're crazy enough to try cutting up the wall. :lol: )
all of which leads back to my tendency to ignore the little elbows. :wink:
P.S. thanks, Deb, for the compliment. i hope i just earned it. :P :?