This is an area I'm somewhat knowledgeable. I've measured thousands of pairs of pants... both inseam and waist. It's best to consider tag sizing as a guideline. Remember, when they make these jeans they have to cut the patterns with allowances to sew them together. They never measure before they sew, so there's natural variation there (+/- an inch as a rule). Patterns are usually cut by machine though, which is a round about way of measuring before cutting.
Levi's - Tag sizes should be considered a guideline. Inseam is usually 2-3" shorter than stated. Waist is usually a bit tighter in tolerance. Not much. Also consider their "shrink to fit" era, which is vintage now and I believe they still make occasional shrink to fit release Levi's (don't quote me on that).
I could go through the brands, but Levi's seems to be a stand out when it comes to larger variances.
Add spandex to the fabric and all bets are off. They'll fit well out of the dryer but toward the end of the day they'll sag. Hate that spandex wave too...
Tell me which brand of jeans and I can tell you my experience.
Some brands also use "vanity sizing".... they want you to feel good wearing their products and suddenly you fit into a Large when you were an XL to begin with. Tommy Bahama comes to mind with vanity sizing. At least their camp shirts....
