For me, it depends on the nib size I'm using. I generally use m10x.75 for #6, and M9x.75 for #5. I'm assuming you are concerned about fit and finish causing leakage issues? I haven't made many eyedroppers, but for piston fillers, which are kind of the same concept (a huge load of ink behind the nib), I fit an o-ring to the base of the section threads by turning a small groove in the threaded tenon. Additionally I turn a recess in the end of the barrel where the section threads in, so that the o-ring fits nicely when the section is screwed into the barrel, making a nice seal.
I know some people simply will use silicon grease to seal any gaps between the barrel and section, so my method may be overkill, however it doesn't take much effort to do, so I'd rather be safe then sorry! I hope this helps you. If my explanation didn't make sense, I'll try and find a picture of this on a pen I've made.
-John
P.S. Digging the Red Green quote Mike!