If the white line cannot be seen in Illustrator is the file ok?

Without knowing specifics,nobody knows.

"Okay" for what?

If it looks okay to you in Illustrator, then it's okay for viewing in Illustrator.

If the export of it does not look okay in Photoshop at 1:1 or higher zoom, then it's probably not okay for whatever you're going to do with that raster image.

If it's printed to a low-res composite printer, then it may be okay, because the printer may not be able to resolve the whitish pixels.

If it's printed for commercial (color-separated) reproduction, it may not be okay, depending on the scale at which it will be printed, and on other considerations partially described below.

The autotrace routine does not build traps. Typically, when you color-fill cartoon line art manually, you don't make the shapes that define the fills merely "kiss" the black line work, as would the default treatments of a stupid autotrace. The black line work typically overprints the fills, thereby creating printing traps.

Suppose a portion of your cartoon is a hand-drawn closed circle. The black line work is irregular; it varies in width, having been drawn with a marker or a brush. The circle is colored in with a medium green. There are no sloppy gaps in the original between the green and the black.

You scan it and autotrace it. Unless you apply some deliberate care to make it do otherwise, the autotrace is going to create a compound path, filled with black, and with no stroke; and a green simple path which (hopefully) exactly "kisses" (abuts) the black path. Adobe's on-screen antialiasing of the edge where the two colors abut may or may not cause your monitor to display a faint whitish or grayish sliver between the two colors.

Similarly, Photoshop's rasterization of it, or the rasterization of a raster export filter may do the same, and may actually result in some off-color pixels along the edge. (Your description of the scenario kinda raises the question of why you are auto-tracing something that you're then just going to rasterize in Photoshop anyway. Why do that? Why not just work with the scan in Photoshop?.)

So let's leave Photoshop out of the picture and assume you are autotracing it because you want vector artwork. You zoom way in to see if the whitish sliver enlarges. It doesn't, so you assume it's just an aberation of Illustrator's on-screen antialising. And then someone tells you you're in the clear. But are you? Not so fast.

Let's assume the artwork is destined for commercial (color-separated) printing. Further assume the color of the autotraced black is 100% K, and the color of the autotraced green is 100Y 50C. Three inks involved. None of those three inks are shared between the two objects. So even if the paths do, in fact, perfectly abut, there is no "wiggle room" built in for the minor alignment shifts that almost aways do occur on press.

Bottom line: Even if you do determine that the common antialiasing aberations that frequently occur on-screen in Adobe apps is just that—just an onscreen aberation, that does not necessarily mean your file is suitable for commercial color-separated reproduction.

First, you need to understand that autotracing is not the one-click, instant "conversion" of a raster image to vector artwork that far too many think it to be. Just like everything else, you don't just launch a program like Illustrator, start autotracing things willy-nilly without understanding what's really going on. Just like anyting else, you can use an autotrace feature intelligently or...well...not.

You have options. Illustrator provides an auto-trapping feature. Read up on it in the documentation so you understand what it's all about. Alternatively, you can expand the results of your autotrace, select all the black linework and apply a composite color that includes 100% K and reasonable percentages of C, M, and Y (a so-called "rich black"). Or,depending on the artwork and the desired results, you may consider doing the autotrace as centerlines so you have stroked paths, not just filled paths for the linework. That way, using the flood fill (so called LivePaint) will cause the auto-created fill objects to extend to the paths, not just to the edges of their strokes. Then set the linework to overprint.

At any rate, if you are doing this professionally, you need to read up on the principles and practices of trapping and color separation.

JET