This sort of depends on the line. Thick, slowly applied lines will blow out a bit more than thin, lightly inscribed ones, which tend to drop out instead. Sculpted lines have less blowout and dropout but will expand slightly with time. A minimum space should probably be between 1/16 and 1/8" depending on these variables- 1/16" is thin but if the lines are light they should maintain their spacing.
I recently watched the new Big Meas lettering DVD, really nicely done and you can get a good sense of his line spacing. He does most of his accent lines in a light pass using a small needle working off the point, moving pretty quickly. Lines like this will settle pretty light and couldn't be counted on for important structural elements, but for accents and flourishes they are just right, and should maintain their spacing because of a lack of blowout.
(note: we have the dvd at the TatEd store)
Need technical support? Call (413) 585-9134 or email