i think if you rip off the selvedge edge itself, you will have a straight edge.
how you then cut that second edge determines whether THAT edge is straight.
huh?
rip off the selvedge, measure in the amount you want. either rip or cut at that measure. if it smiles, then you cut it crooked. the selvedge is always straight. if it's crooked, you can always straighten it out by recutting or re-ripping on the edge that you originally cut. don't touch the selvedge edge.
if you only ripped it, it's always straight.
if you decide to rip that second edge, it may stretch a little. press it out.