Its really not a very specific question that I could give an answer to because every project I do is so very different and working with such a huge range of materials causes it to be always changing, especially being a prototype fabricator because its more sometimes about the look and relatively close tolerances for testing with great efforts to keep the cost of time and money down.
A good example is the Top Speed bike I am building. The tubes on that bike are first rolled with a slight curve then some how with my tricky ways, machines, a bit of magic and wacky bending dies, I am able to put tight bends on the long curved tube. This kind of bending is very complex in a lot of ways both in measuring and in manufacturing.
The odds of hitting exact numbers that Solidworks is giving me on the blueprint is next to impossible, I am close but never exact, it then become more visual and getting the look and flow of the frame then getting the exact numbers ( I would have a pile of bad chromoly tubes touching the moon if I tried to get it perfect like the computer model )
But when it comes to other projects for clients of the aerospace, medical, and science world, the level of expectations are very high so I then turn the attention to the numbers and getting it near or exactly what the customer expects.
Thats my job, and I think I do a good job at it !
Some projects come in that almost make me cry looking at the blue prints, because they expect near perfect things ( most of the time it seems these are not important things needing perfection and sometime are just errors on blueprints )
Learning from the old dudes over the years has stoked me out and stressed me out ( more on the stressed ) because there is those days where you build for days, weeks and months on a project only to find out things moved while welding, machining or handling and now you gotta fix them.
Some of the ideas we come up with to fix them scare or surprise me, thats when the years of experience come in...
Thats will be a great blog post of another day...
On a normal day to day I find myself holding very tight tolerances of measurements mostly because I make a lot of laboratory stuff where mishaps can throw everything off. Some times the worst looking weld seems to be the straightest and strongest and other days when you don't drink 15 cups of coffee and 2 cans of Mt. Dew at 5 AM before welding, things look great and are some times straight too.
Either way the lesson is " Metal loves to move " ( people in to having fun in large groups while drinking a lot of beer ( if you read that without the punctuation, it covered 2 lessons in one))