If you tear the fabric, there is no way it will be crooked, but as stated above, you have to tear both ends. The fabric will only tear on the straight of grain. If you want it cut, though, you should speak up and tell them. You are the customer - and you are always right!
Even if they cut it at the store, when I get home, the first thing I do is tear it to make sure it is straight! Of course, I always buy a quarter yard extra just in case I have miscalculated.