Never, EVER cut all of the parts of a project first, then assemble.
Why? No matter how great you think you are in the shop, there is a good chance you will be off just ever so slightly on a cut or measurement.
And, unless you plane all of your lumber to the same exact thickness, you could run into variations in the size of stock you purchase.
This isn’t to say that you shouldn’t follow measurements in project instructions, but you should always measure distances between parts just to make sure, and then make adjustments as needed.
Being slightly off on project parts, particularly if you cut several identical parts, can create compounded measurement problems. Just being off 1/16 inch on 4 parts can create a 1/4-inch discrepancy.
In conjunction with this – always factor in the kerf, or thickness of a saw blade, when cutting multiple parts from one board. Kerf can vary between circular blades, but they also are different between band saw blades, jig saw blades, etc.
I always allow for a 1/8-inch kerf between cuts when determining a cut list and cutting diagram – both for rip cuts and cross cuts. This is VERY important because if you don’t factor in that fraction, you could come up short on a project part.
For example, you won’t get two 48-inch parts from on 8-foot (96-inch) board. Subtract the 48 inches, then the kerf, and you’re left with no more than 47 7/8 inches. And, if you are checking your board ends for square, and need to trim them up (which you should do anyway), you’ll have even less stock with which to work.
There are rare exceptions when you’ll wind up with a board that is slightly longer than the listed dimension. I’ve had that happen a few times with outdoor lumber. I’ve also had the opposite happen, where a board was shorter than its listed dimension. It can’t be said enough – measure twice and cut once.