Oops I forgot to answer your second part.

To square up my fabric. I'll hold both selvages together with my arms extended so I can see the bottom of the fold. Then I'll shift or slide one selvage edge left or right to make sure the fold is straight with no visible wobbles. Once I see it's straight, I lay it on my table then trim the edge I'm cutting from.

Here's a Youtube vid on what I mean. Penny on the video does it a little differently (which I've done at times too), but is still very effective and will give you the same results.

http://www.youtube.com/watch?v=C_euef3T7b8