For a number of years now, I have assumed that usable width of fabric is 40", no matter what the end of the bolts reads. But, more recently I've been looking at the selvedges as well as what's listed on the bolt. I recently had a student who had fabric with selvedge printing that measured 1" on one side and holes extending 3/4" into the fabric on the other end. That's 1-3/4" of unusable fabric. Her actual usable fabric was 38-3/4". Because of this, she was unable to get the required number of sub-cut units per strip, had to cut extra strips and she had LOTS of "wasted" rectangle ends. She was lucky to have purchased LOTS of extra fabric because she really liked it.
Fabric requirements for newer patterns are usually based on 40" wide fabric, but it is always wise to check for sure. A number of years ago I would buy only what was called for on the pattern, but soon realized that all patterns and fabrics are not created equal. Until I become familiar with a specific designer and his/her "generosity" with fabric requirements, I always buy at least 1/4 yard extra. When I write patterns, I base them on 40" of usable fabric and throw in a bit extra to allow for frequent clean-up cuts, etc.