It gets personal. How close is close enough TO YOU? For me, I am a little OC and will accept .001 and sometimes .002 over in some cases - and round over the ends the .002 oversized ends. I don't accept .002 undersize. I shoot for as close as I can get to .000. Did I mention that I am a little OC?
On some pens with center band designs, .001 or .002 differences are hard to feel, but on pens with smooth transitions on the end cap, nib end and center bands, the hand can tell .002 differences easily.
Now if you want to sell a slimline for $30.00 - $40.00 then .002 or .003 may not be a problem, but if you want to sell the slimline for $75.00 or more, MOST prospective buyers paying that price will inspect it for feel and quality, and that will be for perfectly smooth transition from the blank to the nib to the center band etc.