not all fabric is woven evenly. ripping will usually reveal the true straight of grain, but not always. it always should, but it doesn't always. price is no reliable indicator, either. look at the fabric from the back. if it "eyeballs" even, it'll most likely tear that way. if it doesn't rip to a fairly square end, or if the weave is loose or otherwise wacky, prewash the fabric in hot water and chuck it into the dryer on the highest setting.
whether your rip first or not, press the fabric flat, all the way across from selvage to selvage. lay it out flat, then fold selvage to selvage. don't use the ends as your guides. use the selvages. when they line up together down the length of the fabric and you have a nice flat, even fold with no ripples, you've found what the fabric "considers" its center. press that. lay the fold precisely along a horizontal grid line on the cutting mat. line the ruler up precisely along the fold and the perpendicular (vertical) line closest to the end and cut there. use that as your reference for future cuts.
if it's any consolation, unless you're working with stripes, checks, plaids or any other print on an obvious grid, not being on the perfect straight of grain won't cause major problems or look horrible. like Norah, when i work with fabrics like that, i don't cut the fabric while it's folded. i line up the ruler using the print itself as my guide and move the ruler along as i cut from one selvage to the other in stages.
is that a pain? yes.
is it worth it? also ... yes
and ... because i'm lazy ... if i'm having an elbowy day ... i just don't use the wonky pieces that span the fold. they go into the scrap bag. they'll eventually be good for something. :wink: