Ahh...an age old question asked again and again...
I have found that you need to aline your selvedge edges not the cut edges because the fabric is not straight on the bolt.
This can end up being a couple of inches off on the end of the fabric piece...much to our frustration (because that gets cut off in squaring up fabric)
So, take your selvidge edges and line those up so the fold in the fabric is "nice and correct" then square up your fabric, then cut your strips.
Hope this helps.
Kirsten