I've got a fairly high end 3d printer and could probably print fine enough for pcb resist. The trouble I see is keeping everything at the same temperature so that the material would stick. I wouldn't bother at all with PLA, I'd go with ABS because it can be dissolved with acetone and seems to print more accurately. I've printed things that had to have things dissolved off after, and had to switch filaments etc. It's definitely possible, but realize that you'd probably have to print more than one layer just to fill in the gaps. There might also be a need to bake the board afterward to really insure a bond to the copper. Also most very thin and flat things will warp and pull away from the substrate unless everything is exactly the right temperature. A super heated board and two layers could make a stable print. I've done down to .1mm nozzle, but the actual print "line" is thicker than that as the point of 3d printing is to "squish" the filament to the last layer for proper adhesion. I could easily imagine this working for through hole, but not necessarily fine pitched smt. I'm currently doing experiments myself in making SMT stencils with a cheap chinese laser engraver/cutter. I still rely on either good old photo boards, or for the really heavy stuff(50+amps) milling the board. Although for almost any prototype I still use wirewrap gear and point to point soldering. I'd love to have a "real" pcb printer and may have one in my very near future. For those that don't know these are cutting/laminating machines that create multilayer boards on demand just like a printer. In the mean time I've got a couple identical epsons that need some modification for direct board prints.