Is there anything to the process of 3D printing that might make the dice unfair/land on a specific side more frequently? Or will any imperfections be negligible, thus making the dice fair? I'm more concerned about making a d20 than anything else, so focus on that shape if need be.
I am using a MakerBot Replicator+ with FDM, so take this into consideration if it affects your answer, and if you believe a different printing process would have an advantage over this don't hesitate to include it.
It probably won't be fair.
Incidentally, I have a decent amount of experience with 3D printing.
It depends heavily on the particular technology that you're using to do the 3D printing, but nearly all forms of printing aren't perfect--depending on the design you make (solid? honeycombed? hollow?), there will be slight, or not-so-slight variations across the faces, which will result in an unfair die. In particular, the faces that are parallel to the build surface will probably be different compared to the ones that are angled.
You could probably deal with some of these problems with some kind of post-processing, but honestly at that point buying dice would be easier. 3D printing is for making custom or prototype objects, and isn't really suited to mass production of simple shapes like dice.
Even manufactured dice have some imperfections, and a slightly unfair die probably won't make much of a difference in gameplay, but it will be hard for you to determine whether the imperfections from 3D printing are severe enough to make the die unfair without rolling it many times and looking at its distribution. Again, at that point, it's easier to just buy the dice.
That being said, custom d20s can be super cool looking, and if you're going to 3D print something, might as well make it look cool...