Only you know what’s acceptable for your projects.
Maybe I have old eyes or just bad eyes, but I can’t distinguish between the marks of a 64th of an inch. I have a ruler marked to 1/64th, and I’ve tried countless times to use it, but frankly I can never tell exactly where one tiny mark ends and the next begins. Some will disagree with me, but if you can’t distinguish it with 20/20 vision (corrected or otherwise) then it is very unlikely anyone looking at your project will ever even suspect. I’ve watched some woodworking shows where guys will put on magnifying goggles to read a precision ruler. I admire those guys; I have to admit I was very intimidated by them when I first started learning advanced techniques. I felt I could never achieve that level of perfection. The truth is I haven’t achieved it, nor do I ever expect to. It is best to learn from those guys, watch their skills and techniques but don’t be intimidated by them — find your own comfort level and improve upon it as your skills improve.
For me, I want my projects to be as perfect as possible (Are there levels of perfection? – you know what I mean). If you don’t create a standard that is acceptable to you, you’ll likely never finish a project measuring yourself by someone else’s standard. I use overall appearance and consistency as my basic standard. My eye is naturally more critical of my work, so if it doesn’t look nice to me then I’m going to make corrections to fix it. One method I use is consistency. If you can’t measure each piece to the exact size use guides and jigs to make them all the same. Then if one is off by a 64th or even a 16th (I know!) then they are all off equally and the project will appear perfect.